Instructions

• Preheat oven to 350°F. Line a 9×13-inch pan with foil and coat the foil with cooking spray (or grease the pan).
• In the bowl of an electric mixer fitted with a paddle attachment, beat ½ cup (113 g) unsalted butter and 2 cups (400 g) brown sugar together on medium speed until light and fluffy, about 2–3 minutes.
• Add 2 large eggs and 1 tablespoon vanilla extract; beat until combined, scraping down the bowl as needed.
• Add 1 teaspoon baking powder, ¼ teaspoon salt, and 2 cups (242 g) all-purpose flour. Mix on low speed just until the dough comes together and no large streaks of flour remain.
• Transfer the mixer bowl to a larger bowl (or stop the mixer) and fold in the mix-ins: 1 cup (170 g) mixed white and semi-sweet chocolate chips, ½ cup (toffee bits), and ¾ cup Reese’s Pieces, reserving ¼ cup Reese’s Pieces to sprinkle on top.
• Press the sticky dough evenly into the prepared 9×13 pan. (Spray your hands with cooking spray or lightly grease your hands to prevent sticking.)
• Evenly sprinkle the reserved ¼ cup Reese’s Pieces over the top of the dough.
• Bake at 350°F for about 25 minutes, until the edges are set and golden and the center is slightly soft but not raw. Be careful not to overbake—the center will firm up as it cools.
• Remove from oven and cool completely in the pan on a wire rack. Once cooled, lift out using the foil and cut into squares.