Preheat oven to 350F.
Line 8-by-8 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 M&Ms all over the top.
Lightly push them into the batter.
Bake 20- 22 minutes.
Cool and drizzle with the melted Chocoley Chocolates.