Preheat your oven to 350°F (175°C) and grease the two 9-inch round cake pans with butter or non-stick spray.
In a large mixing bowl, sift together the flour, cocoa powder, sugar, baking powder, baking soda, and salt.
Add the eggs, milk, vegetable oil, bourbon, and vanilla extract to the dry ingredients. Beat the mixture using an electric mixer on medium speed for about 2 minutes until well combined.
Carefully stir in the boiling water until the batter is smooth. The batter will be thin; this is normal.
Divide the batter evenly between the prepared cake pans.
Bake in the preheated oven for 25 minutes, or until a toothpick inserted in the center comes out clean. Allow cakes to cool in pans for 10 minutes, then transfer to a wire rack to cool completely.
While the cakes are cooling, prepare the blackberry buttercream. In a large bowl, beat the softened butter until creamy with an electric mixer.
Gradually add the powdered sugar, beating until combined. Add the blackberry puree and vanilla extract, mixing until smooth. If the buttercream is too thick, add milk a little at a time until you reach your desired consistency.
Once the cakes are completely cool, place one cake layer on a serving plate. Spread a thick layer of blackberry buttercream on top. Add the second cake layer and frost the top and sides of the cake with the remaining buttercream.
Decorate with fresh blackberries if desired. Slice and serve!