Preheat oven to 350˚F. and lightly spray a 9x9 baking dish.

In a large bowl mix together, melted butter, cocoa powder sugar and vanilla. Add in eggs and yolk, stirring until combined.

In a second bowl combine flour, baking powder and salt. Slowly add the butter mixture into the flour mixture and mix until completely combined. Hand crush about 12 Oreo cookies and fold them into the batter.

Put about half of the batter into the prepared baking dish and even out. Add a layer of peanut butter cups, then the rest of the batter. Crush the remaining cookies over the top of the batter.

Bake for about 35 to 40 minutes, until the center is set.