Preheat oven to 350°
Line 8x8 pan with nonstick foil.
Melt the butter in the microwave. Let cool a bit and add sugar and vanilla.
Add egg.
Stir in flour.
Add Oreo chunks.
Spread batter in pan and smooth top with offset spatula.
Sprinkle Reese's Pieces all over the top.
Lightly push them into the batter.
Bake 20- 22 minutes.
Cool and cut into bars or hearts! Enjoy!