Preheat your oven to 350°F (175°C) and prepare three 4-inch cake pans by lining the bottoms with parchment paper and spraying with cooking spray.

In a large mixing bowl, combine the flour, sugar, cocoa powder, baking powder, and salt. Whisk until evenly blended.

Add melted butter, vegetable oil, egg, buttermilk, and vanilla extract to the dry mixture. Whisk until smooth and silky.

Divide the batter evenly among the prepared pans, smoothing out tops. Bake for 32-35 minutes.

Let the pans cool on a wire rack for about 10 minutes, then transfer to the freezer for 20 minutes.

For the frosting, beat room temperature butter until creamy, then add vanilla and salt. Gradually mix in powdered sugar, cocoa powder, heavy whipping cream, and melted chocolate until smooth.

Once layers are cool, level tops if necessary, stack and frost the layers.

Decorate with personal touches like sprinkles, chocolate shavings, or fresh berries. Chill in the refrigerator for 30 minutes before serving.