First, preheat your oven to 350°F (175°C) and grease and line your cake pans with parchment paper. This step ensures that your cake layers release easily after baking. In a medium bowl, sift together the flour, cocoa powder, baking soda, and salt. Sifting is important because it helps create a lighter, fluffier crumb in your velvet cake.

In another bowl, whisk together the buttermilk, blackberry puree, vinegar, and vanilla extract. Set this wet mixture aside. In a large bowl, beat the granulated sugar with the softened butter and oil until light and fluffy. This process incorporates air into the batter, helping the cake rise beautifully. Add the eggs, one at a time, beating well after each addition.

Next, alternate adding the dry ingredients and the wet mixture to the butter mixture, starting and ending with the dry ingredients. Mix just until combined to avoid overworking the batter, which could make the cake tough. At this point, if you want a deeper, gothic hue, add black food coloring a little at a time until the desired color is achieved.

Divide the batter evenly between the prepared pans and smooth the tops. Bake for 25-30 minutes or until a toothpick inserted into the center comes out clean. Allow the cakes to cool in the pans for 10 minutes, then turn them out onto a cooling rack to cool completely.

While the cakes cool, prepare the frosting. Beat the softened butter until creamy, then gradually add the powdered sugar. Once incorporated, add the blackberry puree and vanilla extract, beating until the frosting is smooth and fluffy. If you want a more dramatic finish, add a few drops of black or purple gel food coloring.

Once the cake layers are completely cool, level them if necessary. Place one layer on a cake stand, spread a generous layer of frosting over the top, then add the second layer. Frost the entire cake with a thin crumb coat, chill for 20 minutes, then apply the final coat of frosting, smoothing the sides and top. Decorate as desired with piped rosettes, fresh blackberries, or a dramatic drizzle of blackberry syrup for an extra gothic touch.