Preheat oven to 350°F (175°C). Grease and flour a bundt pan.
In a large bowl, beat butter and granulated sugar until light and fluffy.
Add eggs one at a time, beating well after each addition. Stir in vanilla extract.
In another bowl, whisk together flour, baking powder, baking soda, salt, and cocoa powder.
Gradually add dry ingredients to the butter mixture, alternating with buttermilk, beginning and ending with dry ingredients.
In a separate bowl, beat cream cheese, powdered sugar, and vanilla extract until smooth.
Pour half of the cake batter into the prepared bundt pan. Spoon the cream cheese mixture over the batter, then top with the remaining cake batter.
Bake for 55 minutes or until a toothpick inserted into the center comes out clean.
Cool in the pan for 10 minutes, then turn out onto a wire rack to cool completely.