Preheat oven to 350. Butter a bundt.
Shift together the flour, baking powder, baking soda, and salt. Set aside.
Cream the butter and add the sugar, beat until the mixture is light and fluffy (longer than you think about 5 minutes). Add beaten eggs, one at a time, mix after each addition. Beat in the vanilla and sour cream.
With the mixer on low speed alternately add the flour mixture and the milk to the batter, in three additions, beginning and ending with the flour.
After preparing the batter, pour half of the batter into a separate bowl. Stir the melted chocolate into one half of the batter, mixing well. Place the batter into the prepared pan by alternating spoons of vanilla batter and chocolate batter. Then, with the end of a wooden spoon, gently draw swirls through the batter to marbleize it. Don't over mix or you won't have that wonderful marble affect.
Bake for about 50 - 60 minutes, or until a toothpick inserted in the center comes out clean. Remove from oven and place on a wire rack to cool for about 10 minutes before removing the cake from the pan to cool completely. Serve warm or at room temperature.
bittersweet or semisweet chocolate, chopped
1 tbsp coffee
2 1/4 cups flour
1 tbsp baking powder
1/2 tsp baking soda
1/4 tsp salt
1/2 cup unsalted butter
1 1/4 cups granulated whitesugar
2 tsp vanilla
3/4 cup sour cream
1/3 cup milk