Lightly butter the bottom and sides of an 8×8-inch square pan. Line the pan with a piece of parchment paper, allowing the paper to overhang two opposing sides. This overhang will assist with removing the bars once set. Butter the parchment as well for ease of removal.

In a medium saucepan, melt ½ cup of unsalted butter over low heat. Once melted, remove from heat and whisk in the powdered sugar until fully incorporated and smooth. Add the peanut butter and graham cracker crumbs. Stir until the mixture is homogenous and smooth. Transfer the mixture into the prepared baking dish and spread it evenly using a spatula or the back of a spoon.

In the same saucepan (wiped clean if necessary), melt ¼ cup of unsalted butter over low heat. Add the milk chocolate chips and stir continuously until nearly melted. Remove from heat and continue stirring until the mixture becomes completely smooth and glossy. Pour this mixture over the peanut butter layer, spreading evenly to form a smooth top layer.

Refrigerate the assembled bars for at least 30 minutes, or until the chocolate layer is firm to the touch. Once chilled, remove the bars from the pan using the parchment overhang and transfer to a cutting board. Using a sharp knife or pizza cutter, slice into 16 equal squares. Serve chilled or allow to rest at room temperature for a few minutes before serving.