Place rack in the center of oven then preheat to 350 degrees. Coat muffin tin with non-stick baking spray or arrange cupcake liners then set aside.

Add melted butter to a medium-sized mixing bowl then stir in sugar. Add eggs and vanilla then mix until completely incorporated.

Sift cocoa, flour, salt and baking soda into bowl containing the wet ingredients then fold until completely combined.

Divide batter equally into each muffin cup, filling each one slightly less than ½ full. Bake 12–14 minutes or until barely done or even slightly underdone in the center. Remove from oven then let cool for 5 minutes.

While brownies cool, stir together peanut butter, powdered sugar and vanilla until smooth. Use the handle of a wooden spoon to press a 1" round indentation into the center of each brownie. The indentation should be just big enough to hold about 1 tsp of the peanut butter mixture.

Fill each indentation with an equal amount (about 1 generous tsp) of peanut butter filling. Sprinkle the top of the brownie with semi-sweet and peanut butter chips and return to the oven for about 2 minutes, just to melt the chips a bit. Don’t worry if you’ve turned the oven off, it just needs to be warm.

Remove from oven and cool completely before removing from the pan.