user31974: It's a variation of the Falkbeer countergambit that I'd like to learn to surprise my opponents but I can't find any resource on the internet.
[FEN ""]
1. e4 e5 2.f4 d5 3. exd5 c6

The only videos I find on the Falkbeer countergambit talk about 3... e4 or 3... exf4.
Whether it's books or videos I don't really mind, but I have a small preference for videos.

user15478: This is the analysis I have done with help of the Lichess database.
It is a very sharp variation where Black often sacrifices material for activity.
[FEN ""] 
[ECO "C30"]
[Opening "King's Gambit"]


1. e4 e5 2. f4 c6 3. Nf3 { Played in 2/3 of the games on Lichess. } (3. Nc3! { This is a good move. If you see this move, you can assume that White knows what they are doing. I advise to transpose into KGA, since White has commited to Nc3 and ...c6 is useful to play for ...d5. } 3... exf4 4. Nf3 Be7 { What follows is a sampl line. } 5. Bc4 d5 (5... Bh4+ { Is also possible. It is a variation of the Cunningham Defense. }) 6. exd5 Nf6 7. dxc6 Nxc6 8. d4 O-O 9. Bxf4 Bg4 10. O-O Bxf3 11. gxf3 Qxd4+ 12. Qxd4 Nxd4)  (3. fxe5 Qh4+ 4. g3 Qxe4+ { White is losing. }) 3... d5 4. Nxe5 (4. exd5 e4 5. Ne5 Nf6 { This is my own suggestion, it is tricky to handle for White. } 6. dxc6 Nxc6 7. Bc4 (7. Bb5 Bc5! { Ignoring the threat of Nxc6! Why? If White does not commit to Nxc6, they now cannot castle and the game is easy for Black. } 8. Nxc6 bxc6 9. Bxc6+ Bd7 10. Bxa8 Bg4 { And the Queen is gone. })
7... Bc5! { Is the complicated way. } 8. Bxf7+ Kf8 { It is on White to prove that they can untangle this situation. Here is the best try for White. } 9. Qe2 Nxe5 10. fxe5 Bg4 11. Qf1 Qd4 12. exf6 Kxf7 { The White King is not happy. }) 4... dxe4 5. Bc4 { Note that with this move, the knight on e5 has no retreating squares. } (5. d4 Nh6 6. Nc3 f6 7. Bc4 { Transposes to the insane variation no one will ever play after 6. Nc3. }) 5... Nh6 6. O-O { You need to know how to react to this. } (6. Qh5 g6 7. Nxg6 fxg6 8. Qe5+ Qe7 9. Qxh8 Nd7 { Where Black will play to gain tempi on the Queen and ideally trap her. })  (6. Nc3!? f6 7. d4 { White calls the bluff... No one will ever play this. We cannot afford to take the knight now. } 7... Bf5 8. O-O Nd7 9. Kh1 Qe7 10. Nxd7 Qxd7 11. d5 O-O-O) 6... f6 7. Qh5+ g6 8. Nxg6 Bg4! 9. Bf7+ Nxf7 10. Qxg4 hxg6 11. Qxg6 Qd4+ 12. Kh1 Rh6 *


user4418: Hanging Pawns is a YouTube channel I particularly like because it seems to be aimed at average club players, like me, and run by an average club player who aspires to be stronger.
He has done a video on the Falkbeer Counter Gambit in which he considers three black continuations, 3...exe4 (which he says transposes to the Modern Defence to the King's Gambit Accepted), 3...c6 (the Nimzowitsch Counter Gambit which you are asking about) and 3...e4 (the Staunton variation which he considers visually frightening but harmless if white knows what they are doing).
He spends about a third of the roughly 20 minute video on this line which he considers black's most aggressive and challenging option in the Falkbeer Counter Gambit. It is the best online resource for this variation I have found.

user32165: I'm very confused by the existing answers. This variation has a name. It's called the Nimzowitsch or Nimzowitsch-Marshall Countergambit. According to Wikipedia it's even the most popular response to white's 3. exd5 and in fact is considered to be the best move.
According to Chess Notes it was first played by Marshall against Teichmann in 1905.
According to this article and BCO2 the main line runs 4. Nc3 exf4 5. Nf3 Bd6 6. d4 Ne7 7. dxc6 Nbxc6 8. Bc4 0-0 9. 0-0.
As for videos, any video on the Falkbeer really should cover this variation, although I found some which don't. But here's an example (from 6:50).

31974: <p>It's a variation of the Falkbeer countergambit that I'd like to learn to surprise my opponents but I can't find any resource on the internet.</p>
<pre><code>[FEN &quot;&quot;]
1. e4 e5 2.f4 d5 3. exd5 c6
</code></pre>
<p>The only videos I find on the Falkbeer countergambit talk about 3... e4 or 3... exf4.</p>
<p>Whether it's books or videos I don't really mind, but I have a small preference for videos.</p>
 15478: <p>This is the analysis I have done with help of the Lichess database.
It is a very sharp variation where Black often sacrifices material for activity.</p>
<pre><code>[FEN &quot;&quot;] 
[ECO &quot;C30&quot;]
[Opening &quot;King's Gambit&quot;]


1. e4 e5 2. f4 c6 3. Nf3 { Played in 2/3 of the games on Lichess. } (3. Nc3! { This is a good move. If you see this move, you can assume that White knows what they are doing. I advise to transpose into KGA, since White has commited to Nc3 and ...c6 is useful to play for ...d5. } 3... exf4 4. Nf3 Be7 { What follows is a sampl line. } 5. Bc4 d5 (5... Bh4+ { Is also possible. It is a variation of the Cunningham Defense. }) 6. exd5 Nf6 7. dxc6 Nxc6 8. d4 O-O 9. Bxf4 Bg4 10. O-O Bxf3 11. gxf3 Qxd4+ 12. Qxd4 Nxd4)  (3. fxe5 Qh4+ 4. g3 Qxe4+ { White is losing. }) 3... d5 4. Nxe5 (4. exd5 e4 5. Ne5 Nf6 { This is my own suggestion, it is tricky to handle for White. } 6. dxc6 Nxc6 7. Bc4 (7. Bb5 Bc5! { Ignoring the threat of Nxc6! Why? If White does not commit to Nxc6, they now cannot castle and the game is easy for Black. } 8. Nxc6 bxc6 9. Bxc6+ Bd7 10. Bxa8 Bg4 { And the Queen is gone. })
7... Bc5! { Is the complicated way. } 8. Bxf7+ Kf8 { It is on White to prove that they can untangle this situation. Here is the best try for White. } 9. Qe2 Nxe5 10. fxe5 Bg4 11. Qf1 Qd4 12. exf6 Kxf7 { The White King is not happy. }) 4... dxe4 5. Bc4 { Note that with this move, the knight on e5 has no retreating squares. } (5. d4 Nh6 6. Nc3 f6 7. Bc4 { Transposes to the insane variation no one will ever play after 6. Nc3. }) 5... Nh6 6. O-O { You need to know how to react to this. } (6. Qh5 g6 7. Nxg6 fxg6 8. Qe5+ Qe7 9. Qxh8 Nd7 { Where Black will play to gain tempi on the Queen and ideally trap her. })  (6. Nc3!? f6 7. d4 { White calls the bluff... No one will ever play this. We cannot afford to take the knight now. } 7... Bf5 8. O-O Nd7 9. Kh1 Qe7 10. Nxd7 Qxd7 11. d5 O-O-O) 6... f6 7. Qh5+ g6 8. Nxg6 Bg4! 9. Bf7+ Nxf7 10. Qxg4 hxg6 11. Qxg6 Qd4+ 12. Kh1 Rh6 *
</code></pre>
 4418: <p><a href="https://www.youtube.com/channel/UCkJdvwRC-oGPhRHW_XPNokg" rel="nofollow noreferrer">Hanging Pawns</a> is a YouTube channel I particularly like because it seems to be aimed at average club players, like me, and run by an average club player who aspires to be stronger.</p>
<p>He has done a <a href="https://www.youtube.com/watch?v=wDfwRtjqlug" rel="nofollow noreferrer">video on the Falkbeer Counter Gambit</a> in which he considers three black continuations, 3...exe4 (which he says transposes to the Modern Defence to the King's Gambit Accepted), 3...c6 (the Nimzowitsch Counter Gambit which you are asking about) and 3...e4 (the Staunton variation which he considers visually frightening but harmless if white knows what they are doing).</p>
<p>He spends about a third of the roughly 20 minute video on this line which he considers black's most aggressive and challenging option in the Falkbeer Counter Gambit. It is the best online resource for this variation I have found.</p>
 32165: <p>I'm very confused by the existing answers. This variation has a name. It's called the Nimzowitsch or Nimzowitsch-Marshall Countergambit. <a href="https://en.wikipedia.org/wiki/King%27s_Gambit,_Falkbeer_Countergambit" rel="noreferrer">According to Wikipedia</a> it's even the most popular response to white's 3. exd5 and in fact is considered to be the best move.</p>
<p>According to <a href="https://www.chesshistory.com/winter/winter75.html#CN_6792" rel="noreferrer">Chess Notes</a> it was first played by Marshall against Teichmann in 1905.</p>
<p>According to <a href="http://gambitopenings.blogspot.com/2016/03/destroying-falkbeer-counter-gambit.html" rel="noreferrer">this article</a> and <a href="https://en.wikibooks.org/wiki/Chess_Opening_Theory/1._e4/1...e5/2._f4/2...d5" rel="noreferrer">BCO2</a> the main line runs 4. Nc3 exf4 5. Nf3 Bd6 6. d4 Ne7 7. dxc6 Nbxc6 8. Bc4 0-0 9. 0-0.</p>
<p>As for videos, any video on the Falkbeer really should cover this variation, although I found some which don't. <a href="https://www.youtube.com/watch?v=wDfwRtjqlug" rel="noreferrer">But here's an example</a> (from 6:50).</p>
