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

In a large mixing bowl, whisk together the all-purpose flour, granulated sugar, cocoa powder, baking powder, baking soda, and salt.

In a separate medium bowl, whisk together the buttermilk, vegetable oil, eggs, and vanilla extract until fully combined.

Pour the wet ingredient mixture into the bowl of dry ingredients, gently folding them together until just combined.

Gradually stir in the boiling water, mixing carefully until the batter becomes smooth and thin.

Evenly divide the batter between the prepared cake pans and bake for 30–35 minutes.

Once baked, allow the cakes to cool in the pans for about 10 minutes, then invert them onto a wire rack to cool completely.

For the frosting, cream softened unsalted butter in a mixing bowl until fluffy and pale.

Gradually add the powdered sugar and cocoa powder, mixing on low speed, then pour in the milk and remaining vanilla, beating until smooth.

Level the cooled cakes if necessary, spread a layer of fudge frosting on top of one layer, then add the second layer.

Frost the sides and top of the cake with the remaining fudge frosting, and garnish with chocolate shavings, fresh berries, or chopped nuts.

Refrigerate the finished cake for at least 30 minutes before serving.