In a large saucepan, over medium-low heat, melt the butter and marshmallows, stirring until smooth, 6 to 8 minutes. Add the cereal and stir until coated and combined. Press into a 9-by-13-inch baking dish.
Roughly chop the chocolate and place it in a heatproof bowl set over (but not in) a saucepan of gently simmering water. Spread the chocolate evenly over the rice mixture.
Cover and refrigerate 45 minutes. Cut into 24 squares.