Start by lining a 9x13 inch baking pan with parchment paper.
In a large microwave-safe bowl, combine 8 tablespoons of unsalted butter and 10 ounces of mini marshmallows. Microwave on high for about 1 minute, then stir. Continue to heat in 30-second increments until the mixture is melted and smooth. Stir in 1 teaspoon of vanilla extract.
Pour the melted mixture over 9 cups of Rice Krispies cereal in a large mixing bowl. Gently fold the cereal into the marshmallow mixture using a spatula until well coated.
Fold in the remaining 10 ounces of mini marshmallows.
Pour the mixture into the prepared baking pan. Using a spatula, press the mixture evenly into the pan.
In a separate microwave-safe bowl, melt the 8 ounces of chocolate candy melts according to package instructions. Stir until smooth.
Once the Rice Krispie mixture is set (about 30 minutes), drizzle the melted chocolate over the top.
While the chocolate is still warm, sprinkle the Mini M&Ms over the top. Press them gently into the chocolate.
Allow the chocolate to cool and set completely before cutting into squares.
