Preheat your oven to 350°F (175°C). Lightly grease a 9x13-inch baking pan with cooking spray or butter, then line it with parchment paper, leaving an overhang on the long sides to create a sling for easy removal.
In a medium bowl, combine the graham cracker crumbs, granulated sugar, and fine sea salt. Pour in the melted butter and stir until all the crumbs are evenly moistened and the mixture feels like damp sand. If the mixture looks too dry, add 1–2 more tablespoons melted butter.
1 1/2 cups graham cracker crumbs, 1/2 cup unsalted butter, 1/4 cup granulated sugar, 1/4 teaspoon fine sea salt
Pour the crumb mixture into the prepared pan. Use the bottom of a measuring cup or a flat glass to press the mixture firmly and evenly across the bottom, making sure to press well into the corners so the crust holds together.
1 1/2 cups graham cracker crumbs, 1/2 cup unsalted butter, 1/4 cup granulated sugar, 1/4 teaspoon fine sea salt
Slowly pour the sweetened condensed milk evenly over the graham cracker crust. Tilt the pan gently or use a small offset spatula to nudge it into an even layer without disturbing the crust.
14 ounces sweetened condensed milk
Sprinkle the semi-sweet chocolate chips evenly over the condensed milk layer. Then sprinkle the butterscotch chips and any optional white chocolate or peanut butter chips evenly over the top.
1 1/2 cups semi-sweet chocolate chips, 1 cup butterscotch chips, 1/2 cup white chocolate chips or peanut butter chips
Scatter the chopped nuts evenly over the chips. Press them down very gently with your hand so they nestle slightly into the layers. If desired, you can add nuts over only half the pan to keep part nut-free.
1 cup chopped nuts
Sprinkle the sweetened shredded coconut flakes evenly over the entire surface, covering the chips and nuts without creating thick mounds so the coconut browns evenly.
1 1/3 cups sweetened shredded coconut flakes
Bake at 350°F (175°C) for 22–28 minutes, or until the edges are deeply golden brown and the coconut on top is lightly to moderately toasted. The sweetened condensed milk should be bubbling around the edges. Start checking at about 20 minutes. If the coconut is browning too quickly while the center is still pale, loosely tent the pan with foil for the remaining time.
Remove the pan from the oven and place it on a cooling rack. Allow the bars to cool completely in the pan, about 2–3 hours at room temperature. For faster cooling, let cool 30–45 minutes on the counter, then transfer the pan to the refrigerator for about 1 hour. Do not cut while warm or the bars will be very soft and messy.
Once fully cool and set, use the parchment sling to lift the slab of bars out of the pan onto a cutting board. Use a sharp knife to cut into 24–32 bars, wiping the blade with a warm, damp cloth between cuts for clean edges. If desired, sprinkle each bar with a tiny pinch of flaky sea salt before serving.
Flaky sea salt