Melt chocolate and butter together in top of double boiler, then set aside to cool.
Beat egg yolks with sugar and vanilla until creamy and light.
Beat egg whites until stiff.
Finely grind the almonds until almost powdery; the ground almonds are the flour in this cake. You may be able to purchase already ground almonds.
Whisk the finely ground almonds with potato starch and baking powder together.
Add melted butter & chocolate, and beaten egg yolks to the almond mixture with a wooden spoon or spatula.
Gently fold in egg whites with a spatula until just fully incorporated.
Butter the sides and bottom of the cake pan- springform pan is ideal.
Line the bottom of the pan with an 8" round cut from baking paper.
Pour the batter into the 8" baking pan.
Bake at 180 C° or 350 F° for 45 minutes.
Cool for half an hour, remove cake from the pan and place on a serving platter.
Dust lightly with powdered sugar.
Note: This cake freezes well. Defrost at room temperature.