Begin by preheating your oven to 350 degrees and coating two 6-cup jumbo muffin pans with nonstick cooking spray. In a medium bowl, whisk together 2 cups all-purpose flour, 2/3 cup cocoa powder, 1 1/2 teaspoons allspice, 1 teaspoon baking powder, 1/4 teaspoon baking soda, and 1/4 teaspoon salt.
Set this dry mixture aside. In a large mixing bowl, beat 3/4 cup softened unsalted butter for about 2 minutes until creamy. Add 1 1/2 cups light brown sugar and continue beating until the mixture becomes light and fluffy. Incorporate 3 eggs one at a time, beating well after each addition, then beat in 2 teaspoons vanilla extract.
Alternate adding the flour mixture and 1 cup sour cream in three additions each, beating well between additions. After the final addition, beat for 2 minutes more, then fold in 4 ounces chopped bittersweet chocolate. Divide the batter evenly among the prepared muffin cups, filling each about halfway full.
Bake the cakes at 350 degrees for approximately 25 minutes, or until the tops spring back when lightly touched. Remove from the oven and allow the cakes to cool for 10 minutes on a wire rack before carefully turning them out onto the rack to cool completely.
While the cakes cool, prepare the ganache by bringing 1 cup heavy cream to a boil in a small saucepan. Place 8 ounces chopped bittersweet chocolate in a medium bowl and pour the hot cream over the top. Let it sit for a moment, then stir until the mixture is completely smooth and glossy.
Once the cakes have cooled completely, place a wire rack over a jelly roll pan and arrange the cakes flat-side down on the rack. Pour 2 to 3 tablespoons of ganache over each cake, using a spatula to smooth it evenly over the top and sides. Transfer the cakes to the refrigerator and allow the ganache to set for at least one hour before serving. At this point, you can decorate the cakes as desired with additional toppings or garnishes.
