In a medium-sized bowl, combine the butter, crumbs, sugar, and peanut butter and stir to combine. (Or you can alternatively mix in a food processor.)
Press into the casserole pan into an even layer.
In a microwave-safe bowl, place the chocolate chips and heat in 25-second bursts. Stir in between each heating until fully melted.
Pour the melted chocolate over the peanut butter layer and smooth out with an offset spatula.
Place in the refrigerator to set for 2 hours or overnight.
Cut into 9 squares and serve or package.
Notes
Leftover bars will keep in an airtight container in the refrigerator for one week. They are also great for freezing, and will keep in the freezer for 3 months.