Start by preheating your oven to 350 degrees and lining a baking pan with parchment paper.
In a food processor, combine the black beans, cocoa powder, oat flour, peanut flour, stevia syrup, and non-dairy milk. Blend until completely smooth. The batter should be thick but spreadable. Add an extra tablespoon of milk if needed.
Fold in chocolate chips if you are using them, then spread the batter evenly into your prepared pan.
Spoon the peanut butter over the top of the batter. Use a knife or toothpick to swirl it through the surface for that classic marbled look.
Bake for 20 to 23 minutes until the center is just set. Let the brownies cool completely before slicing. This step is key for the perfect fudgy texture.