Preheat oven to 350°F.
Line a 11 x 13 metal or glass baking pan with parchment paper.
In a large bowl, mix the peanut butter, melted butter, white sugar, and brown sugar together. Slowly add the eggs, vanilla extract, and pinch of salt.
In a separate bowl, whisk the flour and baking powder.
Slowly add the flour and baking powder mix to the peanut butter mixture and mix or fold to combine. DO NOT OVER MIX!
Smooth the batter into the 11 x 13 pan and sprinkle the Reese's Pieces over the batter.
Bake for 30-35 minutes. They will feel firm on top and moist in the center.
Allow them to cool for at least 30 minutes before cutting them.