Preheat oven to 350°F (175°C). Grease and flour two 9-inch round cake pans.

In a large bowl, whisk together flour, sugar, cocoa, baking soda, baking powder, and salt.

Add eggs, buttermilk, vegetable oil, bourbon, and vanilla extract. Beat on medium speed for 2 minutes until well combined.

Stir in the boiling water by hand. Note that the batter will be thin.

Pour batter evenly into the prepared pans. Bake for 30 to 35 minutes, or until a toothpick inserted in the center comes out clean.

Cool in the pans for 10 minutes, then transfer to wire racks to cool completely before frosting.

To prepare buttercream, beat softened butter for 5 minutes until pale and fluffy.

Gradually add confectioners sugar, one cup at a time, beating well after each addition.

Add the reduced blackberry puree, 1 tablespoon bourbon, and salt. Whip on high speed for 2 minutes until light and airy.

Frost the cooled cake layers, starting with a layer of buttercream in the center and finishing with the top and sides.