Begin by greasing the bottom of an 8-inch square baking pan with cooking spray and lining it with parchment paper, ensuring you leave a 2-inch overhang on two opposite sides for easy removal.

In a large heatproof bowl, combine the rice cereal and rolled oats.

In a small saucepan over medium-low heat, bring the brown sugar, corn syrup (or honey), unsalted butter, smooth peanut butter, and kosher salt to a low simmer. Stir continuously until the sugar is completely dissolved, about 3 minutes.

Pour the gooey concoction over the cereal mixture and stir gently to combine into a crispy, sticky mixture.

Transfer this mixture into your prepared baking pan. Using the back of a measuring cup, press it firmly into the pan to create an even, compact layer.

Refrigerate until the bars are cold and set, about 2 hours or overnight.

Once set, lift the bars onto a cutting board using the parchment overhang and slice them into 12 rectangles.

In a small heatproof bowl, microwave the semisweet chocolate chips and coconut oil in 30-second increments, stirring in between until melted and smooth.

Dip the bottom of each bar into the melted chocolate, then place them on a parchment-lined baking sheet.

Transfer remaining melted chocolate into a plastic bag, seal it, snip a tiny hole in the corner, and drizzle chocolate over the bars.

Sprinkle mini M&Ms on top and refrigerate until the chocolate is set, about 30 minutes.