Step 1

Preheat oven to 350 degrees F (175 degrees C). Grease 2 8x8-inch baking pans.

Step 2

Melt unsweetened chocolate and butter in the top of a double boiler over simmering water; stir to mix well. Beat the eggs and egg whites together in a mixing bowl with an electric mixer on high speed until very fluffy, about 5 minutes. Gently stir the butter and chocolate mixture, sugar, coffee, and vanilla flavoring into the egg mixture, and beat well to combine.

Step 3

Mix together all of the potato starch, all of the matzo cake meal, cocoa powder, and salt in a bowl. Beat the dry ingredients into the chocolate mixture with electric mixer until fluffy and thoroughly combined. Gently fold the chocolate chips into the batter, and divide between the 2 prepared baking dishes.

Step 4

Bake in the preheated oven until a toothpick inserted into the center of a pan shows moist crumbs, 25 to 30 minutes.