Preheat the oven to 350°F (175°C) and grease a 9x5-inch loaf pan or similar cake pan.

In a large mixing bowl, cream the softened butter and sugar until light and fluffy, about 3-4 minutes.

Beat in the eggs one at a time, then mix in the vanilla extract. Gradually add the flour, milk, and baking powder, mixing until just combined.

Split the batter in half, placing one half in another mixing bowl. Stir the cocoa powder into one half until fully incorporated.

In the prepared pan, alternate spoonfuls of the vanilla and chocolate batter, swirling them together with a knife or skewer to create a marbled effect.

Bake in the preheated oven for 30-35 minutes, or until a toothpick inserted in the center comes out clean.

Allow the cake to cool in the pan for about 10 minutes before transferring it to a wire rack to cool completely.

For the buttercream frosting, beat together softened butter, powdered sugar, milk, and vanilla extract until creamy.

Once the cake is cool, spread the buttercream frosting generously over the top and sides of the cake.

Slice and enjoy your delicious creation!