Instructions

• Preheat oven to 350°F. Spray a 9x13-inch pan very well with cooking spray (or line with nonstick foil and spray the foil); set the pan aside.
• In a large bowl combine 2 ⅓ cups all-purpose flour, ⅔ cup packed light brown sugar, and ½ teaspoon salt. Add ¾ cup unsalted butter (1 1/2 sticks), very well softened. Using a fork or pastry cutter, cut the butter into the dry ingredients until the mixture resembles sandy crumbs with a few larger pea-sized crumbs. (It will seem dry at this point; that is normal.)
• Add 1 large egg and ½ teaspoon vanilla extract to the bowl. Continue mixing with the fork (or a spoon) until the egg and vanilla are incorporated and the dough comes together.
• Stir in 1 cup semi-sweet chocolate chips and ½ cup peanut butter chips until evenly distributed.
• Measure and remove 1 1/2 cups of this chip-and-dough mixture and set it aside. Reserve it for the topping.
• Transfer the remaining chip-and-dough mixture into the prepared 9x13 pan. Use your hands or the back of a measuring cup to press and pack it into a smooth, even layer to form the crust.
• Bake the crust for 10 minutes at 350°F (rotate the pan once midway through baking if desired). Remove the pan from the oven.
• Immediately and evenly drizzle the entire 14-ounce can of sweetened condensed milk over the warm crust.
• Evenly sprinkle the reserved 1 1/2 cups of chip-and-dough mixture over the sweetened condensed milk, breaking it up slightly with your fingers if needed so it covers the surface.
• Evenly sprinkle 1 cup of the toffee bits over the top.
• Evenly sprinkle the remaining 1 cup semi-sweet chocolate chips and remaining ½ cup peanut butter chips over the top.
• Return the pan to the oven and bake for 25 to 30 minutes at 350°F, or until the top looks set in the center and a few large bubbles intermittently appear on the surface. Rotate the pan midway through baking if desired. (The bars will seem loose when hot but will firm up as they cool.)
• Remove the pan from the oven and immediately sprinkle the remaining ¼ cup toffee bits evenly over the top.
• Allow the bars to cool in the pan at room temperature for at least 90 minutes before slicing. For best results, let them cool 2–3 hours or overnight (cover if leaving overnight).
• Store bars airtight at room temperature for up to 1 week or freeze for up to 4 months.