Step 1

For the cake: Preheat the oven to 350 degrees F. Position a rack in the center of the oven. Grease the cake pans with 1 tablespoon butter, then dust with 1 tablespoon cocoa powder, tapping out any extra. Set aside.

Step 2

In a medium bowl, sift together the flour, baking powder and remaining 1 1/2 cups cocoa powder. Set aside.

Step 3

In the bowl of a stand mixer fitted with the paddle attachment, cream the sugar and remaining 3 sticks butter on medium speed until light and fluffy, 5 to 8 minutes. Add the vanilla, then beat in the eggs one at a time. On low speed, add the salt directly to the wet ingredients, then add half of the flour mixture and mix until just combined. Add the sour cream and mix until just combined. Remove the bowl from the mixer and gently incorporate the remaining flour mixture with a rubber spatula. Do not overmix. Fold in the chocolate chips.

Step 4

Spread half of the batter in each of the prepared pans. Tap the pans lightly on the counter so that the batter settles. Place the pans on the center rack of the oven and bake until a tester or small knife inserted in the center emerges clean, 40 to 50 minutes. Cool for at least 30 minutes. Unmold onto a rack. Cool to room temperature, 1 1/2 to 2 hours more.

Step 5

Mix together the bourbon and the simple syrup in a bowl. Set aside.

Step 6

For the frosting: In a medium pot, bring the heavy cream to a simmer. In a medium oven-safe bowl, combine the chocolate and sugar. Place the bowl on top of the pot with the heavy cream to melt. Once the cream is simmering, pour it over the chocolate mixture and stir until all of the chocolate has melted and you have a glossy ganache. Gently stir in the vanilla, followed by the butter pieces. Set aside to let cool to room temperature.

Step 7

Transfer the chocolate mixture to the clean bowl of a stand mixer fitted with the whisk attachment. Whip the frosting for 1 to 2 minutes to lighten the texture.

Step 8

Put the cake layers on a baking sheet fitted with a wire rack. Using a pastry brush, brush about half the bourbon mixture on each cake layer. Frost the top of the bottom layer, then stack the other layer on top. Frost the top and sides of the cake.