Preheat oven to 350 degree F.
Lightly butter an 8" square pan and line with parchment paper leaving a 1" hang on either side to use as "handles" to remove blondies after baking.
In a medium sized bowl whisk together the melted butter and both sugars. Add egg and vanilla and whisk until mixture is completely lump free.
Add the flour and salt to the batter, folding in with a rubber spatula until flour is barely visible. Do not over mix.
Mix in one cup of the peanut butter cup pieces and fold gently.
Spread batter evenly into lined, greased pan and top with remaining ½ cup of peanut butter cup pieces.
Bake for approximately 40 to 45 minutes, or until edges are lightly browned, the center is set and a toothpick inserted into the center comes out clean.
Let fully cool for at least 2 hours before lifting from pan using the parchment edges and cutting with a large chefs or serrated knife into 16 squares. Makes 16 squares