Adapted from a MarthaStewart.com recipe.
9 tablespoons unsweeted cocoa powder
3 tablespoons sugar
1 ½ cups AP flour
½ cup whole wheat flour
1 teaspoon baking powder
1 teaspoon baking soda
1 teaspoon salt
1 stick butter, room temperature
1 cup sugar
2 large eggs, room temperature
1 cup sour cream (or Greek style plain yogurt)
1 teaspoon vanilla
Preheat oven to 350. Thoroughly butter and flour a bundt pan.
Make marble filling: stir together cocoa powder and 3 T sugar in a small bowl; set aside.
Make cake batter: Whisk together next five ingredients. Cream together butter and 1 cup sugar. Beat in eggs one at a time, then vanilla. Mix in flour mixture, alternating with sour cream.
Spread about one third of batter in bundt pan. Sprinkle half of filling. Spread another third, then other half of filling, then final third of batter. Swirl batter and filling slightly with a knife or skewer.
Bake 35 – 40 minutes, until a tester comes out clean. Cool 30 minutes, then invert onto a plate.