Instructions

• Preheat oven to 350°F. Line an 8x8-inch baking pan with aluminum foil and spray the foil with cooking spray; set the prepared pan aside.
• In a large, microwave-safe bowl, melt ½ cup (1 stick) unsalted butter on high for about 1 minute. Let the butter sit 20–30 seconds so it cools slightly.
• Add 1 large egg to the warm (not hot) butter, then add 1 cup packed light brown sugar, heaping 1/3 cup creamy peanut butter, and 1 tablespoon vanilla extract. Whisk until smooth and well combined.
• Add 1 ¼ cups all-purpose flour and a pinch of salt (optional). Stir with a spatula until just combined; do not overmix.
• Stir in 3/4 cup peanut butter chips until evenly distributed. Mix just until incorporated so the chips keep their shape.
• Turn about two-thirds to three-quarters of the dough into the prepared pan and use a spatula to spread and press it evenly into a crust that reaches the edges. Reserve the remaining dough in the bowl.
• In a medium, microwave-safe bowl, add the remaining 1 cup peanut butter chips and pour half of one 14-ounce can of sweetened condensed milk over the chips. Microwave on high in 30-second intervals, stirring between intervals, until the chips have melted and the mixture is smooth (about 1–2 minutes total). Stop and stir every 30 seconds to prevent scorching.
• Immediately pour the melted peanut butter mixture evenly over the pressed crust and smooth the top lightly with a spatula. Work quickly—the mixture begins to set after a few minutes.
• Crumble the reserved dough over the top of the peanut butter layer. Use heaping 1-tablespoon-sized blobs and then gently press and smooth them together with a spatula so the surface is mostly covered (it’s okay if there are some small gaps).
• Bake in the preheated oven for 27–29 minutes, or until the top is just set and very pale in color. A darker top will yield firmer, crumblier bars; for soft bars, keep the color pale.
• Allow the bars to cool in the pan for at least 1 hour before lifting out on the foil and slicing. Store bars airtight at room temperature for up to 1 week or freeze for up to 6 months.