I'm not a vegetarian but I don't want to use gelatin.  I have seen various recipes for making mousse but they seem to be the gooey type. I'm looking for a recipe which will set so I can make a mousse cake which can be free standing.  This picture is similar to what I'm looking for:    I have a stand mixer and am happy to use eggs, double cream, etc. and to experiment, but don't want to make a massive amount. I can use agar but so far am always lost as most recipes which call for gelatin cannot easily be converted for use with agar. 
About two years ago I read about the "ultimate chocolate mousse" from Heston Blumenthal. Interestingly, the recipe only calls for two ingredients: bittersweet chocolate and water. Sugar can be added, but it is optional. It is all in the technique.  You use an approach that is similar to tempering chocolate and then whip.  Place a mixing bowl over a bowl filled with iced. Melt 265g bittersweet chocolate (chopped) with 240 ml water over medium heat. Gently stir to keep the temperature consistent throughout the chocolate. You pour the melted chocolate into the mixing bowl sitting on the bowl with ice in it and start whisking. Here you will need to may need a bit of practice. If you mix too much, it will become grainy. If that happens, then lightly melt again and then pour back into the cooled bowl. Chocolate can be very fluffy even without added ingredients. 