Preheat your oven to 350°F (175°C). Grease and flour the two 9-inch round cake pans.
In a large mixing bowl, combine the flour, cocoa powder, sugar, baking powder, baking soda, and salt. Mix well.
Add the eggs, milk, vegetable oil, bourbon, and vanilla extract to the dry ingredients. Beat on medium speed for about 2 minutes until well combined.
Carefully stir in the boiling water until the batter is smooth. It will be thin.
Divide the batter evenly between the prepared cake pans. Bake for 30 minutes or until a toothpick inserted in the center comes out clean.
Allow the cakes to cool in the pans for about 10 minutes, then transfer them to a cooling rack to cool completely.
While the cakes are cooling, prepare the blackberry buttercream. In a mixing bowl, beat the softened butter until creamy.
Gradually add the powdered sugar, mixing on low speed, followed by the blackberry puree, heavy cream, vanilla extract, and salt. Beat until fluffy and well combined.
Once the cakes have completely cooled, spread a layer of blackberry buttercream on top of one cake layer, then place the other layer on top.
Frost the top and sides of the cake with the remaining blackberry buttercream.
Optional: Decorate the cake with fresh blackberries and chocolate shavings.