Preheat oven to 350 degrees F. Grease or line an 8 x 8 baking pan, set aside. In a small mixing bowl combine the flour, baking powder, and salt, stir to combine and set aside.
In the bowl of a hand or stand mixer fitted with a paddle attachment, cream the butter and sugars until light and fluffy, 3-4 minutes. Add in the peanut butter and mix for an additional 1-2 minutes. Add in the eggs, one at a time and the vanilla. Scrap down the sides of the bowl.
Slowly add in the dry ingredients and mix until combined. Fold in the chopped mini chocolate chips and Reese's peanut butter cups, transfer into the prepared baking pan. If desired, press a few whole Reese's cups on top prior to baking.
Bake at 350 degrees for 25-30 minutes or until a toothpick inserted comes out clean. Remove form the oven and let cool on a wire rack, sprinkle with flaky salt and allow to cool before cutting.