Grease a 9x9-inch baking pan with butter or line it with parchment paper.

Melt 4 tablespoons of butter in a medium saucepan over low heat. Add both mini marshmallows and stir until smooth, about 5–7 minutes. Mix in lemon zest and lemon extract.

Remove from heat and gently fold in 6 cups of Rice Krispies cereal.

Transfer mixture into the prepared pan and press down evenly with a buttered spatula.

Melt white chocolate chips in a microwave-safe bowl, then drizzle over the pressed mixture.

Cool at room temperature for at least 30 minutes before cutting into squares.