Preheat oven to 350 degrees. Line a 9x13-inch metal baking pan with foil, set aside.
In a medium saucepan set over medium heat, combine butter, peanut butter and brown sugar. Stir until the mixture is completely melted. Remove from the heat and stir in the vanilla. Transfer the mixture to a large mixing bowl. Allow to cool for 5 minutes before adding the eggs, one at a time, mixing well after each addition.
In a medium mixing bowl whisk together the flour, salt and baking powder. Gradually add dry ingredients into the wet ingredients. Mix until only a few streaks of flour remain, then add peanut butter chips and peanut butter cups. Fold until combined.
Spread the thick batter into the prepared baking pan, smooth the top. Add additional chips and peanut butter cups to the top for a "decorative" touch, if desired.
Place in the oven and bake for 20-24 minutes or until the blondies are set. Do not over-bake. Remove from the oven and allow to cool in the pan for 20 minutes before cutting and serving.
Notes
* I used mini peanut butter cups from Trader Joe's which are much smaller than the Reese's mini peanut butter cups.