Preheat the oven to 350F and grease and line an 8?8 square baking dish with parchment paper.

In a measuring glass, combine the butter and chocolate. Microwave in 30 second intervals until smooth.

In a large bowl using a hand or stand mixer with the whisk attachment, mix together the sugars and egg on high speed until pale and super smooth (about 3 minutes).

Turn the mixer down to medium-low and trickle in the melted chocolate while mixing. Mix in the vanilla.

Pour in the flour, cornstarch, cocoa powder, and salt. Use a rubber spatula to mix the batter by hand, followed by the chocolate chips. Set aside.

In a medium bowl, combine the flour, oats, baking powder, and salt for the monster cookie dough.

In a large mixing bowl using a hand or stand mixer with the paddle attachment, mix together the peanut butter, butter, and sugars until smooth and well combined.

Add in the vanilla and egg and mix on medium-high speed for about 2 minutes.

Pour in the dry ingredients for the monster cookie dough and use a rubber spatula to mix, followed by the chocolate chips and Reese?s pieces.

In the baking pan, alternate adding spoonfuls of brownie batter and cookie dough, then spread everything evenly.

Sprinkle with a handful of chocolate chips and extra Reese?s pieces.

Bake for 30-35 minutes. When the bars are fresh from the oven, stick little candy eyes to the melted chocolate chips on top.

Allow the bars to cool for about 45 mins ? 1 hour, then lift out, slice and enjoy!