Preheat your oven to 350°F (175°C). Grease and flour a baking pan.
In a mixing bowl, cream together the butter and sugar until light and fluffy.
Add the eggs one at a time, beating well after each addition.
Mix in the vanilla extract.
In another bowl, combine the flour, baking powder, and salt. Gradually add to the creamed mixture alternately with milk, beating well after each addition.
Divide the batter in half. Stir cocoa into one portion. Drop alternating spoonfuls of plain and chocolate batter into the prepared pan. Swirl with a knife for a marbled effect.
Bake for 45 minutes or until a toothpick inserted into the center comes out clean. Cool in the pan for 10 minutes before removing to a wire rack to cool completely.