Preheat the Oven: Start by preheating your oven to 350°F (175°C). Grease and flour your bundt pan.

Cream Butter and Sugar: In a large bowl, use an electric mixer to cream together the softened butter and sugar until light and fluffy.

Add the Eggs: Add the eggs one at a time, mixing well after each addition. Stir in the vanilla extract.

Mix Dry Ingredients: In another bowl, whisk together the flour, baking powder, baking soda, and salt.

Combine Mixtures: Gradually add the dry ingredients to the creamed mixture, alternating with the milk until just combined.

Divide the Batter: Divide the batter in half. To one half, stir in the cocoa powder.

Create the Marble Effect: Spoon alternating dollops of the vanilla and chocolate batters into the prepared bundt pan and swirl gently with a knife.

Bake: Place the pan in the oven and bake for 50-60 minutes, or until a toothpick inserted comes out clean.

Cool the Cake: Let the cake cool in the pan for about 10 minutes, then transfer to a wire rack to cool completely.