Preheat oven to 350 degrees F; prepare 2 loaf pans, spraying with cooking spray and set aside.
Sift flour, baking powder, baking soda and salt in a medium size bowl and set aside.
In a large bowl, with an electric mixer or stand mixer fitted with the paddle attachment, mix butter, sugar, vanilla and eggs until well combined.
Lower mixer speed and add buttermilk until incorporated
Slowly add flour mixture in 3 batches, mixing well after each addition.
Melt chocolate in a double boiler, or slowly in the microwave on the defrost setting.
Divide batter evenly between loaf pans; divide both chocolates and pour slowly over each pan, using a butter knife or fork to swirl chocolate to create marble effect.
Bake for 30-40 minutes, or until a toothpick inserted in the middle of each cake comes out clean.
Cool completely before serving.
Notes and Suggestions
This recipe makes 2 loaves of 12 servings each. Wrap one of the loaves and freeze for an easy, instant dessert to bring along to a party or serve to impromptu guests.