Preheat oven to 350°. Line a 9×13” pan with parchment paper.: You should notice a gentle, warm hum as the oven comes to temperature, and the interior light will make the racks glow faintly. Preheating ensures the crust starts to set immediately when it hits heat so the structure becomes crisp at the edges. A common mistake is slipping into multitasking and placing the pan in too cool an oven, which can make the center soggy instead of firming up, so wait until it reaches the full temperature.
Finely crush cookies in a food processor. Mix with melted butter and press into prepared pan.: The parchment will wrinkle slightly when you press it into the pan but it creates a smooth, nonstick surface that makes removal effortless. Press the paper into the corners and leave a bit of overhang for easy lifting. If you skip this step, the bars can cling to the pan and crumble when you try to remove them, so take the extra minute to line it.
Sprinkle chocolate chips, nuts, peanut butter chips, pretzels, and M&Ms evenly over the top of the crust. Pour the sweetened condensed milk evenly over the top of the bars.: When you pulse the cookies , you should hear a rapid, whispering buzz as they turn into fine crumbs, and the aroma of peanut butter will bloom. Aim for a texture similar to coarse sand so the crumbs compact well. Over processing into powder risks a dry, crumbly crust, while under processing leaves large chunks that won’t bind as well with the butter.
Bake for about 25 minutes, until edges begin to brown. Cool completely before cutting.: The butter should be warm and glossy, coating the crumbs and making them slightly clump together. Use the back of a measuring cup or your fingers to press the mixture firmly and evenly into the pan so the crust is compact and uniform. If you press unevenly, you may get thin spots that become too crisp while the center stays loose, so apply steady pressure across the whole surface.
Sprinkle chocolate chips, nuts, peanut butter chips, pretzels, and M&Ms evenly over the top of the crust: As you scatter the toppings, aim for balance so each square will have a bit of everything. The contrast between glossy chocolate, crunchy nuts , and colorful M&Ms creates visual appeal and varied texture. A common oversight is dumping toppings in clumps near the center, which yields inconsistent bites, so take time to distribute them in an even layer.
Pour the sweetened condensed milk evenly over the top of the bars: Pour in a slow, steady stream so the sweetened condensed milk spreads and fills gaps without washing away the toppings. You want it to sit on top and seep into crevices, creating a shiny sheet that will set into chewy pockets. Pouring too quickly can push lighter toppings aside, and uneven coverage can lead to dry spots in the finished bars, so move steadily and check for gaps.
Bake for about 25 minutes, until edges begin to brown: During baking you will see the edges turn a warm golden color and the syrupy top will tighten and become glossy. The oven will fill with a sweet, toasty aroma as the sugars and butter caramelize. Avoid opening the oven frequently because temperature swings can prevent proper setting, and be careful not to overbake since that will dry out the gooey centers you want to keep soft.
Cool completely before cutting: As the pan cools, the sweetened condensed milk sets into slightly chewy ribbons and the layers firm up enough to slice cleanly. You might notice steam rising for a while and the surface becoming less shiny, which are good signs that the bars are setting. Cutting too soon risks a messy, gooey center that oozes out, so be patient and let them rest until they hold their shape.
