Preheat your oven to 350°F (175°C) and grease a bundt pan.
In a large bowl, cream together the softened butter and sugar until light and fluffy.
Add in the eggs, one at a time, beating well after each addition.
Stir in the vanilla extract.
In another bowl, whisk together the flour, baking powder, and salt.
Gradually add the dry ingredients to the creamed mixture, alternating with the milk.
Divide the batter in half; stir cocoa powder into one half until well combined.
Pour half of the vanilla batter into the prepared pan, followed by the chocolate batter.
Top with the remaining vanilla batter and use a knife to swirl the two batters together.
Bake for 45-50 minutes or until a toothpick inserted in the center comes out clean.