Prepare a baking dish with parchment paper and set aside.
In a mixing bowl, add the melted butter and brown sugar. Whisk together until combined
Then add the peanut butter, vanilla and eggs. Whisk again until the ingredients are blended evenly.
Add the salt, flour and baking powder all at once to the wet ingredients. Fold together until the dry ingredients are incorporated and a thick batter is formed.
Fold in the chopped chocolate bar next. Pour the batter into the prepared baking dish and spread evenly.
Press Reese’s peanut butter cups into dough, creating a 3x3 square of peanut butter cups.
Bake in the oven for 30 minutes or until a toothpick inserted comes out clean.
Let these cool for 20 minutes before removing from the pan.