Line a 9×9-inch baking pan with parchment paper, leaving a little overhang on the sides. Lightly grease the paper with butter or non-stick spray.

In a large mixing bowl, melt 4 tablespoons of unsalted butter in the microwave until just melted (about 30 seconds). Stir in 3 cups of Rice Krispies cereal and 1/4 teaspoon salt. Toss gently to coat the cereal evenly. Set aside.

In a medium saucepan over low heat, melt 2 tablespoons of unsalted butter. Add 10 ounces of marshmallows and stir continuously until melted and smooth (about 5-7 minutes).

Remove from heat. Stir in 1 teaspoon peppermint extract and a few drops of green food coloring (if using). Mix until color is evenly distributed.

Pour the melted marshmallow mixture over the cereal in the large bowl. Fold gently but thoroughly until every bit of cereal is coated.

Transfer the mixture into the prepared pan. Using a buttered spatula or wax paper, press the mixture evenly and firmly into the pan without pressing too hard.

If desired, sprinkle crushed candy canes or drizzle melted white chocolate on top. Press gently to embed toppings slightly.

Let the bars cool at room temperature for about 1 hour until firm. Refrigerate to speed up setting if desired, but avoid freezing.

Use the parchment paper overhang to lift the bars out of the pan. Cut into squares or rectangles with a sharp knife, wiping the knife with a warm damp cloth between slices for cleaner cuts.