Preheat the oven to 350°F. Grease two 6-inch round cake pans and line the bottoms with parchment paper.

In a large bowl, whisk together the flour, granulated sugar, black cocoa powder, baking soda, and salt.

In a separate bowl, whisk the buttermilk, vegetable oil, egg, and vanilla extract until smooth.

Pour the wet mixture into the dry ingredients and stir until just combined. Divide the batter evenly between the two prepared pans.

Bake for 22 to 25 minutes, or until a toothpick inserted in the center comes out clean. Cool the cakes in the pans for 10 minutes, then turn them out onto a wire rack to cool completely.

In a medium bowl, beat the softened butter with an electric mixer until creamy. Gradually add the powdered sugar and black cocoa powder, then beat in the milk until the frosting reaches a spreadable consistency.

Place one cooled cake layer on a plate and spread half the frosting over the top. Add the second layer and spread the remaining frosting over the top and sides.