In a double broiler gently melt the butter and chocolates together
As soon as the mixture is melted, turn off the heat and allow to cool.
Whisk together the flour, baking powder, salt and Cocoa in a bowl.
Beat the eggs with the sugar on high speed (I use my whisks), until the mixture is very pale and thick, and falls back into the bowl with a wide ribbon that folds back on itself and slowly dissolves when the beater is lifted from the surface.
Add the vanilla and beat to combine.
Gently mix in the cooled chocolate mixture into the eggs and sugar
Then mix in the flour mixture 1/2 cup at a time.
Bake at 350 degrees until the top surface is slightly puffed and uniformly dry, and the batter no longer jiggles when you shake the pan gently - about 25-30 minutes depending on how thick they are. They will sink a bit as they cool.