1Preheat oven to 325. Butter a 9 inch square baking pan. Dust pan with cocoa. Put all ingredients in a food processor and process until smooth and well blended, 1 to 2 minutes. Or put in large bowl and beat with mixer at low speed until just blended. Spoon batter into prepared pan. Bake until toothpick inserted into center comes out clean, 50-60 minutes. Cook cake completely in pan on a wire rack. For the frosting, melt the chocolate in double boiler over barely simmering water. Remove from heat. Stir in sour cream and powdered sugar. Do not let the frosting cool completely or it will be too thick to spread. Top with the walnut halves.
Serves 8 to 10
The Golden Book of Chocolate