Preheat no oven is needed. Line two 9-inch pans with foil or parchment and spray the lining lightly with nonstick cooking spray.

Measure 9 cups (237 g) Rice Krispies cereal into a large bowl and set aside.

In a large saucepan, melt 8 tablespoons (113 g) unsalted butter over medium heat.

Add 20 ounces (566 g) mini marshmallows to the melted butter and stir constantly until the marshmallows are completely melted and the mixture is smooth. Remove the pan from the heat.

Stir 1 teaspoon (15 ml) vanilla extract into the melted marshmallow mixture.

Quickly pour the marshmallow mixture over the cereal in the large bowl. Fold and stir until all the cereal is evenly coated.

Divide the coated cereal mixture between the two prepared pans. Using a buttered spatula or a piece of wax paper (spray the wax paper with nonstick spray so it won’t stick to your hands), press the mixture firmly and evenly into the pans to compact it.

Let the Rice Krispie layers cool and set until firm at room temperature (about 20–30 minutes).

Lift the set Rice Krispie slabs from the pans using the foil or parchment and peel the lining off the bottoms. Place the slabs on a cutting board or work surface.

Use a Mickey Mouse–shaped cookie cutter to cut desired shapes from the slabs. Transfer the cut shapes to a wax-paper-lined baking sheet. Save any scraps to reshape or re-press if desired.

Melt 8 ounces chocolate candy melts according to the package directions. (If using a microwave: place melts in a microwave-safe bowl and heat at 50% power in 30-second increments, stirring after each increment, until smooth.)

Dip each Rice Krispie shape into the melted chocolate to coat as desired. Use a fork or dipping tool to lift the treat and let excess chocolate drip back into the bowl.

Immediately press Mini M&Ms (from the 116.9 ounce (family size) bag) onto the chocolate-coated treats so they adhere. Place finished treats back on the wax-paper-lined sheet.

Allow the chocolate to harden completely at room temperature (or refrigerate briefly to speed setting).

Store the finished treats in an airtight container at room temperature for up to 3 days. To freeze: make sure treats are completely cool and set, cut into bars if not already individual, wrap each bar individually in wax paper, place in an airtight container, and freeze for up to 3 months.