Better known as lunch lady bars: a no-bake base of graham crumbs, powdered sugar and peanut butter, under a chocolate layer loosened with more peanut butter so it slices cleanly instead of cracking.
Set aside a fifth of the peanut butter for the topping (4 tablespoons for one pan). In a medium bowl, stir the rest together with the graham cracker crumbs, powdered sugar and melted butter until no dry crumbs or streaks of sugar remain. It should look like damp sand and hold together when you squeeze a handful.
Tip it into an ungreased 9×13 inch pan and press it into a firm, even layer across the bottom. The flat base of a measuring cup gets into the corners better than fingers do.
Put the chocolate chips and the reserved peanut butter in a microwave-safe bowl. Microwave on high in 15-second bursts, stirring after each, and stop as soon as it is smooth and glossy with no whole chips left. Pushed further, the chocolate turns thick and grainy.
Pour the chocolate over the crust and spread it to the edges in an even layer.
Refrigerate for at least 1 hour, until the top is set firm, then cut into 12 squares. A knife dipped in hot water and wiped dry goes through the chocolate without cracking it.
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