Line an 8x8-inch baking dish with parchment paper, leaving an overhang on two opposite sides for easy removal; set aside.

In a medium saucepan over medium-low heat, combine 1/3 cup light brown sugar (packed), 1/4 cup creamy peanut butter, 1/4 cup honey, and 4 tablespoons unsalted butter.

Stir the mixture frequently as it heats. Bring it just to a boil, then continue cooking and stirring constantly for 2–3 minutes, or until the sugar has dissolved and the mixture is smooth.

Remove the saucepan from the heat. Stir in 2 cups quick oats and 1/2 cup crispy rice cereal until the dry ingredients are evenly coated.

Let the coated oat mixture cool in the saucepan for 5 minutes so it is warm but not hot.

If you want M&Ms or other candy from the trail mix on top, pick out a handful from the 1 cup Monster trail mix now and set those pieces aside (optional).

Fold the remaining trail mix into the cooled oat mixture until evenly distributed.

Transfer the mixture to the prepared baking dish. Use a spatula to spread it evenly, then press firmly into an even layer so the bars hold together.

Sprinkle the reserved M&Ms (if any) and 1/4 cup semisweet chocolate chips over the top. Use the bottom of a bowl or cup to press them lightly into the surface so they stay in place.

Refrigerate the dish for 1–2 hours, or until the bars are firm and set.

Use the parchment overhang to lift the set mixture from the dish, place on a cutting board, cut into bars, and serve.