FOR THE CAKE:
  butter (for greasing the cake pans)
1 1/3 cups unbleached all-purpose flour (plus more for dusting the cake pans)
2/3 cup high-quality unsweetened cocoa powder
1 1/2 teaspoons baking soda
1 teaspoon salt
1/4 cup cornstarch
1 cup buttermilk or sour cream
3/4 cup organic or expeller-pressed canola oil
2 large eggs
1 1/2 cups granulated sugar
1 tablespoon pure vanilla extract
 
  FOR THE FROSTING:
8 tablespoons unsalted butter
1/2 cup bittersweet chocolate (coarsely chopped)
1/3 cup high-quality unsweetened cocoa powder
2 cups confectioner's sugar
1/2 cup sour cream
 
  FOR ASSEMBLING THE CAKE:
1/3 cup seedless raspberry jam (optional)
5 cups fresh raspberries (for the filling and garnish, optional)