Position a rack in the center of the oven and preheat to 320ºF.
In a small pot, melt the butter and margarine over medium heat. Once melted, remove the pot from the heat and stir in the chocolate with a rubber spatula until melted, then stir in the egg yolks. Set aside.
Put the egg whites in a very clean bowl and whip until stiff peaks form. Take care not to over whip the egg whites or they will break down. Fold in the chocolate mixture.
Spray the wells of a cupcake tray with nonstick spray. Pour 1/3 cup of the mixture into each well.