Line a 10x15-inch cookie sheet with parchment or wax paper.
In a microwave-safe bowl, melt 1 1/2 cups of the chocolate chips in 15second intervals, stirring well between each cooking interval until melted.
Spread the melted chocolate evenly onto the parchment paper. Place in the refrigerator to chill while you move on to the next step.
Place the peanut butter, powdered sugar, and butter in a microwave-safe mixing bowl, then heat on high in the microwave for 20 seconds; stir until smooth.
Spread the peanut butter mixture evenly over the chilled chocolate layer.
Melt the remaining chocolate chips in the microwave in 15 second cooking intervals, then spread over the peanut butter layer.
Top with Reese's Pieces, then chill the bark in the refrigerator until set (about 60 minutes).
Break the chilled Reese's Peanut Butter Chocolate Bark into individual servings.