Combine the flour, baking powder, and salt and sit it to the side.
Then, using a hand or stand mixer, beat the butter and sugar together on medium speed until light and fluffy (approximately 5 minutes).
Next, add the vanilla and mix.
Add the eggs to the bowl one at a time, mixing until combined before adding the next egg.
Using a spoon or spatula, scrape down the sides of the bowl.
Mix in the flour 1 c at a time, alternating with the buttermilk, beginning and ending with the flour.
Pour half of the mixture into a separate bowl for the swirl.
Add the cocoa powder and milk to the bowl and mix until combined.
Spray a 12-cup bundt pan with baking spray.
Using an ice cream scoop or ¼ cup measuring cup, alternate dollops of vanilla and chocolate cake batter into the bundt pan.
Continue to alternate between plain and chocolate batters until both batters are gone. Use a knife or similar, swirl the batter in an S pattern.
Place the cake in the oven, set the oven to 325 degrees, and bake for about an hour to an hour and 10 minutes. The cake is done once an inserted toothpick comes out clean.
Remove the cake from the oven and allow it to cool for about 10 - 15 minutes before removing it from the pan. Allow the cake to continue to cool on a wire rack.
Serve and enjoy!