In a medium microwave safe bowl, melt the butter on High for 30 to 45 seconds, until melted. Stir in peanut butter and powdered sugar. Add graham cracker crumbs and oats and mix well. Spread into prepared pan.
Melt chocolate chips in microwave on High, stirring every 15 seconds, until melted. Spread on top of peanut butter layer.
Refrigerate for at least 1 hour before cutting into squares. Store in an airtight container in the refrigerator.