Preheat oven to 325 degrees F (165 degrees C). Lightly grease or spray a 10 inch tube pan.
Beat egg yolks until light. Gradually add 1 cup sugar and continue whipping until thick and pale, about 5 minutes. Add the cocoa powder to the egg mixture. Blend.
Sift together the matzo cake meal and potato starch; set aside.
Beat egg whites until foamy. Gradually add remaining 1/2 cup sugar, continuing to beat until whites form stiff peaks.
Add the coffee to the and the dry ingredients to the egg mixture. Stir to mix.
Fold about 1/3 of the egg whites into the batter to lighten the batter. Then fold in remaining whites until no white streaks remain. (If adding some chopped walnuts, fold them in now.) Spread evenly into the prepared tube pan.
Bake for 45 minutes in the preheated oven, until cake springs back to the touch. The top will have small cracks in it. Do not overbake.
Invert the cake onto a wire rack to cool. Run a knife around the sides of the cake before removing from the pan.