Assemble the cake: Place 1 cake layer on a platter; scoop the ice cream on top and spread it to the edge using an offset spatula. Top with the second cake layer, then run the spatula around the sides to smooth the ice cream. Freeze until firm, about 4 hours.
Make the topping: About 30 minutes before serving, remove the cake from the freezer and let stand at room temperature. Toss the berries and 3 tablespoons sugar in a bowl; set aside until ready to serve. Beat the heavy cream, the remaining 1 tablespoon sugar and the vanilla in a bowl with a mixer on medium speed until medium peaks form, 1 to 2 minutes. Spread on top of the cake. Top with the berries and their juices.
Preheat the oven to 350 degrees F. Coat two 9-inch-round cake pans with cooking spray and line the bottoms with parchment paper.
Whisk the cocoa powder and 1 1/2 cups boiling water in a medium bowl until smooth; set aside. Whisk the flour, sugar, baking powder, baking soda and salt in a large bowl until combined. Add the eggs, vegetable oil, sour cream and vanilla and beat with a mixer on medium speed until smooth, about 1 minute. Reduce the mixer speed to low; beat in the cocoa mixture in a steady stream until just combined, then finish mixing with a rubber spatula. (The batter will be thin.)
Divide the batter between the prepared pans and tap the pans against the counter to help the batter settle. Bake until a toothpick inserted into the middle comes out clean, 30 to 40 minutes. Transfer to racks and let cool 10 minutes, then run a knife around the edge of the pans and turn the cakes out onto the racks to cool completely. Remove the parchment. Trim the tops of the cakes with a long serrated knife to make them level, if desired.
Recipe courtesy Food Network Magazine