Grease bottom and the lower 1 inch of the sides of a 9x13-inch baking dish with butter. Pour cereal into the prepared dish.

Mix butterscotch chips and peanut butter together in a bowl and melt in a microwave oven, stirring every 30 seconds, until smooth, 2 to 3 minutes; pour over cereal and stir until cereal is coated. Use a spatula or the back of a spoon to compact mixture into the dish.

Cover the baking dish with plastic wrap and refrigerate until firm, about 10 minutes.

Melt the chocolate in a microwave-safe glass or ceramic bowl in 30-second intervals, stirring after each melting, for 1 to 3 minutes (depending on your microwave). Spread over the top of the bars with a spatula.

Refrigerate bars again until the chocolate is firmed, about 10 minutes.