Preheat oven to 350°. Spray two (9 x 5) loaf pans with nonstick baking spray, or five mini loaf pans (5 ¾ x 3 ¼ x 2 ¼.)
In a medium bowl whisk together flour, baking powder, baking soda and salt. Set aside.
In the bowl of a stand mixer beat the butter and the sugars until light and fluffy, about two minutes. Mix in peanut butter. Add eggs one at a time mixing well after each addition, scraping down sides of bowl as needed. Add vanilla and sour cream and mix to combine. Add flour mixture mixing on low just until blended.
In a medium microwave safe bowl, melt chocolate chips in microwave on 50% power, stirring often. Cool slightly. Stir in 1 ½ cups batter just until blended.
Using a scoop, alternative scoops of the chocolate and peanut butter batters in the prepared pans. Spread the batters so they touch each other and the sides of the pan. Using a butter knife, do a figure 8 through the pan twice. Don’t over mix the batters; you want to create a marbled look, not stir the batters together.
Bake for 50 minutes or until the center springs back when lightly pressed and toothpick comes out clean. (Bake 30-35 minutes for the mini loaves.)
Cool in pan 5 minutes on wire rack. Remove from pan and cool completely on wire rack before serving.