My grandmother made the best biscuits in the world.  She made them 7 days a week, and cooked them on a cast iron skillet without any sides.  They were always perfectly browned and the bread inside was "stretchy" (they were not layered). I've been working on this for several years and I've gotten to this point from my childhood memories and a fair amount of experimentation:  2.5 cups flour. (not sure if she had self rising, but that's not really an option for me in Mexico) around 1/2 cup Crisco shortening (I'm having to use Grasa Vegetal because Crisco isn't available). Cut in until the flour is sort of grainy/crumbly. Pretty sure she just used her fingers from vague memories as a child. 1 cup of buttermilk (Here I use one lemon and fill the cup with whole milk, nata is hard to come by and buttermilk is non-existent in the stores here) 3 Tbsp of baking powder (yea, it's a lot, but I'm literally in a boat at sea level and it seems to need that much) 1 Tsp each of sugar and salt  To get the stretchy part, I replaced part of the "butter" milk with an egg white, then with 2 egg whites, still don't have the stretch I'm looking for. To get the browning I painted the tops with the yellow from the egg that I got the white from. The browning is about the right color but appears painted on, something hers never did.  The stretchy is proving very elusive and I'm curious how to get that.  Her biscuits had a lot of air in them, they were super light, my mother never could reproduce them either. Grandma's been gone for 30 years and even when I asked her to write down the recipe she didn't.  I've come to understand that she was probably illiterate considering when and where she was born and the fact that farm women in the south in the early 1900s were not encouraged to be literate. I do remember she kept her flour in a giant bowl and would mix "in the flour", then lift the dough up onto the powdered counter to cut and finish the biscuits (hence her lack of measurements). What will give me stretch that was close to a sponge cake and yet still leave me with a biscuit?  She had no fancy ingredients available to her but she might have substituted lard for Crisco. 
Miscellaneous thoughts: Egg whites are not going to give you "stretch" in a biscuit. The fat will interfere with that. The elasticity will come from the gluten. That means at least a little bit of kneading; it means not using "cake" or "biscuit" flour; and it means keeping the fat level under control. (Half a cup sounds reasonable, but try it with less, and don't blend it in too finely.) Better gluten formation will also lead to more of a rise. You might try mixing in some tortilla flour, actually. Buttermilk would ordinarily be used with baking soda, not baking powder (or at least, not just with baking powder). Using baking powder and lemon juice together may be pushing the pH too low, weakening the dough. If you're using baking powder, leave out the lemon juice. Egg yolks will always give you a "painted on" glazed look. If you want more of a browned look, put the yolks in the dough, and perhaps increase the sugar. You can also brush the tops with milk, that'll give more of a matte look than egg yolks. 