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

Melt 3 tablespoons of butter in a large saucepan over low heat. Add 4 cups of mini marshmallows and stir until fully melted and smooth, about 3-5 minutes.

Remove from heat and stir in 1 tablespoon of freshly grated lemon zest and 1 teaspoon of lemon extract.

Fold in 6 cups of Rice Krispies cereal until coated evenly, mixing gently for about 1-2 minutes.

Transfer the mixture to the prepared baking pan and press evenly into the pan using a buttered spatula.

Melt 1 cup of white chocolate chips in a microwave-safe bowl, stirring every 20 seconds until smooth. Drizzle over the pressed mixture.

Sprinkle additional lemon zest on top and let the treats cool at room temperature for at least 30 minutes.

Lift the treats out of the pan and cut into squares for serving.