Scoop out 2 ½ cups of the batter, and place it into a separate bowl.
Stir cocoa powder and milk into the 2 ½ cups of batter until well-combined.
2/3 cup sifted unsweetened cocoa powder, 1/2 cup milk
Add remaining 1 teaspoon of vanilla extract to original batter.
Using a 1/4-cup measuring cup, drop one scoop of plain batter into prepared bundt pan.
Using a separate scoop, drop 1/4 cup of chocolate batter on top of plain batter.
Continue to alternate between plain and chocolate batters until both batters are gone.
Give the pan a couple of taps to level out the batter, if necessary. Do not stir.
Bake for 50-65 minutes, or until a wooden toothpick inserted into the center comes out clean.
Cool cake in the pan on a wire rack for 15 minutes.
Invert the cake onto wire rack, and cool completely.
Video
Notes
Butter: I use salted butter in this recipe.
Cake flour: Cake flour helps to make this cake tender. To make a cake flour substitute, you can use this cake flour substitute recipe. Be sure to properly measure the flour to avoid heavy, dry cake. Weighing is the most accurate way to measure the flour, or sift, lightly spoon into a measuring cup, and level.
Buttermilk: You can use regular or low-fat buttermilk in this recipe. However, I recommend using full-fat buttermilk for moister cake. If you would like to make a substitute, you can use this buttermilk substitute recipe.
Milk: I recommend using whole milk for moister cake. You can also use buttermilk instead of milk.