Prepare the pan by greasing an 8x8 inch pan with butter or non-stick spray.

Melt 3 tablespoons of butter and 4 cups of marshmallows in a large saucepan over low heat, stirring constantly until smooth and glossy.

Remove from heat and stir in the zest of one lemon until well incorporated.

Quickly add 6 cups of Rice Krispies to the melted mixture and gently fold in.

Transfer the mixture to the prepared pan and press it firmly to pack tightly.

Melt 1 cup of white chocolate in a microwave-safe bowl in 30-second intervals, stirring in between.

Drizzle the melted white chocolate over the top of the set Rice Krispy base.

Allow to cool completely for about 30 minutes, then cut into squares.