Allow eggs, butter, and sour cream to stand at room temperature for 30 minutes. Meanwhile, generously grease and flour a 10-inch fluted tube pan or tube pan. In a medium bowl, sift together flour, cocoa powder, and baking soda; set aside.

Preheat oven to 325 degrees F. In a large mixing bowl, beat butter with a freestanding electric mixer on medium to high speed for 30 seconds. Gradually add granulated sugar and brown sugar, beating about 10 minutes or until mixture is very light. Add vanilla. Add eggs, one at a time, beating for 1 minute after each addition and scraping sides of bowl frequently. Alternately add flour mixture and sour cream to butter mixture, beating on low to medium speed after each addition just until combined. Pour batter into prepared pan (pan will be full).

Bake for 85 to 90 minutes or until a cake tester or toothpick inserted near the center comes out clean. Cool in pan on a wire rack for 10 minutes. Remove cake from pan. Cool completely on rack. If desired, sprinkle with powdered sugar. Makes 12 servings.

Make-Ahead Tip:Prepare and bake the cake as directed; cool (do not sprinkle with powdered sugar). Wrap in moistureproof and vaporproof wrap or place in an airtight container. Store at room temperature for up to 2 days or freeze for up to 3 months. Thaw the wrapped cake at room temperature for several hours. If desired, sprinkle with powdered sugar.