Preheat the oven to 350°F (175°C)
Grease and flour a loaf pan or bundt pan
Cream butter and sugar until light and fluffy
Add eggs one at a time, mixing well after each
Stir in vanilla extract
In a separate bowl, mix flour, baking powder, and salt
Add dry ingredients to the butter mixture alternately with milk
Mix until just combined
Divide the batter into two portions
Stir cocoa powder and a little milk into one portion
Spoon the vanilla and chocolate batters into the prepared pan in alternating layers
Swirl the batters together gently with a knife
Bake until a toothpick inserted in the center comes out clean
Cool in the pan for a few minutes
Remove from the pan and cool completely on a wire rack