Preheat oven to 350°F (175°C). Grease and flour a 9-inch bundt or loaf pan.
Cream the butter and sugar together. Add eggs, one at a time, beating well after each addition. Stir in vanilla extract.
In a separate bowl, mix flour, baking powder, and salt. Gradually add the dry ingredients to the wet ingredients, alternating with the milk.
Divide the batter into two bowls. Stir cocoa powder into one bowl to create the chocolate batter.
Pour half of the vanilla batter into the prepared pan, then add the chocolate batter.
Use a knife to swirl the two batters together, creating a marble effect.
Bake for 45-55 minutes, or until a toothpick inserted into the center comes out clean. Allow the cake to cool before serving.