1.Preheat oven to 375°F. Mist a 15-by-11-inch sheet pan with cooking spray. Combine 16 Tbsp. butter, 5 Tbsp. cocoa and 1 cup water in a medium saucepan. Bring to a boil over medium-high heat, stirring. Remove from heat.
2. In a large bowl, combine flour, sugar, baking soda and salt. Stir in cocoa mixture, then sour cream and eggs. Spread batter evenly in pan. Bake until a skewer comes out clean, about 22 minutes.
3. To make icing, combine 8 Tbsp. butter, milk and 1/4 cup cocoa in a small saucepan. Bring to a boil over medium-high heat, stirring. Remove from heat; add vanilla. Gradually whisk in confectioners' sugar.
4. Smooth icing onto hot cake; sprinkle with nuts. Cool on a wire rack. Cut into 24 pieces and serve, or chill in an airtight container.