Preheat oven to 400°F.
Butter 9-inch cake pan thoroughly and line the bottom with a round of buttered parchment paper.
Melt chocolate over low heat, preferably in a bain-marie. When the chocolate has melted, remove from heat and stir in the butter until it melts.
Combine the egg yolks and sugar in a mixing bowl and beat until the mixture is fluffy and has lightened to a pale yellow.
Add half of the almond flour and beat until incorporated. Add the rest of the almond flour and beat on medium speed until well combined.
Add the melted chocolate and butter, and vanilla or other flavoring, and beat to combine for 1 minute.
Beat egg whites until stiff and fold in carefully.
Place cake in oven and immediately lower heat to 350°F. Bake for approximately 30-35 minutes. Watch the cake carefully the last 4 minutes. It can go from being alright to being burnt very quickly.
Let cool and remove carefully from pan.