Engel's Passover Brownies

Preheat oven to 325 °F. and butter an 8-inch square baking pan. Chop unsweetened chocolate and in a small heavy saucepan melt with butter over low heat, stirring until smooth. Cool mixture 10 minutes. In a small bowl whisk together cocoa powder and cake meal. In a large bowl with an electric mixer beat together eggs and sugar until thick and pale and beat in sour cream and melted chocolate mixture. Add cocoa powder mixture and beat at low speed until just combined. Stir in chocolate chips and walnuts with a wooden spoon and spread batter evenly in baking pan. Bake brownies in middle of oven 35 minutes, or until a tester comes out with crumbs adhering. Cool brownies completely in pan on a rack before cutting into 16 squares. Brownies keep layered between sheets of wax paper in an airtight container at cool room temperature 5 days.