1 cup unsalted butter (2 sticks), cut into pieces, plus more for the pans
2 cups all-purpose flour, spooned and leveled
1 1/2 cups granulated sugar
3/4 cup unsweetened cocoa powder
1/2 cup dark brown sugar
1 teaspoon baking soda
1 teaspoon kosher salt
2 large eggs
1/2 cup sour cream
1 teaspoon pure vanilla extract
Creamy Chocolate Frosting (make your own, or store bought)
Heat oven to 350 degrees F
Butter two 8- or 9-inch round cake pans and line the bottom of each with a round of parchment paper.
In a large mixing bowl, whisk together the flour, granulated sugar, cocoa powder, brown sugar, baking soda, and salt.
In a small saucepan, combine the butter and 1 cup water and bring to a boil. Add to the flour mixture and, using an electric mixer, mix until combined. Beat in the eggs, one at a time, then the sour cream and vanilla. Beat slowly after each addition.
Divide the batter evenly between the prepared pans. Bake until a toothpick inserted in the center of each cake comes out clean, 35 to 45 minutes. Cool in the pans for 20 minutes, then turn out onto a rack to cool completely.
Transfer one of the cakes to a platter and spread with ¾ cup of the frosting. Top with the remaining cake and spread with the remaining frosting.