Preheat the oven to 350 degrees.

Combine all of the ingredients except for the cocoa powder in a mixing bowl.

Beat on high until smooth and creamy.

Spread ⅔ of the batter into the bottom of a well-greased bundt pan.

In a medium bowl, combine the remaining ⅓ of the batter with the cocoa powder.

Mix until well combined.

Spoon the chocolate batter on top of the mixture in the bundt pan.

Using the tip of a butter knife, swirl the chocolate batter into the vanilla batter.

Bake for 50 minutes, or until cooked completely through.

Allow the cake to cool to room temperature before removing from the bundt pan.