Marble Bundt Cake

CAKE

 * 1 cup butter (at room temperature)1 cup butter (at room temperature)
 * 2 ½ cups granulated sugar2 ½ cups granulated sugar
 * 1 tablespoon + 1 teaspoon vanilla extract (divided)1 tablespoon + 1 teaspoon vanilla extract (divided)
 * 4 large eggs (at room temperature)4 large eggs (at room temperature)
 * 1 cup buttermilk1 (at room temperature)1 cup buttermilk1 (at room temperature)
 * 3 cups cake flour23 cups cake flour2
 * 2 teaspoons baking powder2 teaspoons baking powder
 * 1/2 teaspoon salt1/2 teaspoon salt

CHOCOLATE SWIRL

 * 2/3 cup unsweetened cocoa powder2/3 cup unsweetened cocoa powder
 * 1/2 cup milk3 (at room temperature)1/2 cup milk3 (at room temperature)

INSTRUCTIONS

 * Preheat oven to 350 F. Grease and flour a 12-cup Bundt pan. Set aside.
 * In the bowl of a stand mixer fitted with a paddle attachment (or with an electric mixer), beat together butter and sugar until well-combined.
 * Add eggs one at time, mixing well after each addition.
 * Stir in 1 tablespoon vanilla until combined.
 * Mix in buttermilk until well-combined.
 * Add cake flour, baking powder, and salt.
 * Stir in on low just until incorporated.
 * 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.
 * 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 50-70 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.