Make the chocolate batter by combining the flour, brown sugar, cocoa powder, baking soda, baking powder, and salt together in a large bowl; stir until mixed well.
In a separate bowl, combine the sour cream, vegetable oil, egg, and vanilla extract together; whisk very well until creamy and smooth.
Pour the wet ingredients into the bowl with the dry ingredients and mix with a rubber spatula until just combined.
Pour the chocolate batter into the prepared loaf pan and smooth it out with a rubber spatula.
Pour the peanut butter batter on top of the chocolate batter.
Use a table knife to gently swirl the two batters together.
Sprinkle the top with chocolate chips right before baking.
Place into the oven and bake for 45-55 minutes, or until a tester inserted in the center comes out clean.
Remove from the oven and allow to cool in the pan for 15 minutes before transferring to a cooling rack.
Slice and serve. Enjoy.