Preheat oven to 325 degrees.
Spray bundt pan with baking spray. Set aside.
Stir together flour, baking powder, and salt. Set aside.
Cream butter at high speed of an electric mixer.
With mixer running add sugar, a little at a time, and continue beating until fluffy, about 2-3 minutes, scraping the sides and bottom of the bowl if necessary.
Add eggs, one at a time, beating well after each addition.
Add flour mixture alternately with milk, beginning and ending with flour mixture.
Stir in vanilla.
Spoon 1/4 of cake batter into melted chocolate, and fold in completely.
Spoon half of remaining batter into in bundt pan.
Top with chocolate batter.
Pour the rest of the plain batter on top of the chocolate batter.
Using the handle of a wooden spoon or a knife, swirl the batters together.
Bake until cake pulls away from the sides of the pan and tests done, about 60-65 minutes.