Preheat the oven to 350°F (175°C). Grease and flour the two 9-inch round cake pans.
In a large mixing bowl, combine the flour, sugar, cocoa powder, baking powder, baking soda, and salt. Whisk until well combined.
Add the eggs, milk, vegetable oil, bourbon, and vanilla extract to the dry ingredients. Mix with an electric mixer on medium speed for about 2 minutes.
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 two prepared cake pans. Bake in the preheated oven for 30 minutes, or until a toothpick inserted in the center comes out clean.
Remove from the oven and let the cakes cool in the pans for about 10 minutes. Then, transfer the cakes to a cooling rack to cool completely.
While the cakes are cooling, prepare the blackberry buttercream. In a large mixing bowl, beat the softened butter with an electric mixer until creamy.
Gradually add the powdered sugar, alternating with the blackberry puree and heavy cream. Continue to beat until the mixture is fluffy and smooth. Add vanilla extract and a pinch of salt, mixing well.
Once the cakes have cooled, place one layer on a serving plate. Spread a generous amount of blackberry buttercream on top. Place the second layer on top and frost the top and sides of the cake with the remaining buttercream.
Decorate with fresh blackberries if desired. Slice and serve.