Preheat oven to 350. Line a mini-muffin tin with silicone liners.
Combine all of the ingredients in a blender or food processor, except for the chocolate chips, walnuts and peanut butter. Blend just until smooth. Fold in 1⁄4 cup chocolate chips and walnuts (optional).
Fill each muffin well with batter. Top with remaining mini-chocolate chips and spoon a small spoonful of peanut butter on the top of each brownie.
Bake in oven for 15-17 minutes. Allow to cool. Store in the refrigerator.