Line a 9 by 13 inch baking dish with parchment paper (see photos in the post) or spray with cooking spray. In a large pot, combine the butter and marshmallows over medium-low heat. Stir until the mixture is melted and smooth. Remove from the heat.
6 cups mini marshmallows, 3 tablespoons unsalted butter
Stir in the lemon extract and lemon zest. Then stir in the cereal.
1 teaspoon lemon extract, 1 tablespoon lemon zest, 5 1/2 cups puffed rice cereal
Spray a piece of parchment (or wax) paper with cooking spray and gently press the mixture into the pan, being careful not to push down too hard.
While they are cooling, melt the white chocolate chips according to the directions. Once melted, stir in the vegetable oil to thin the chocolate. Using a fork, drizzle the chocolate over the Rice Krispie Treats. Add a little more lemon zest to the top.
1/2 cup white chocolate chips, 3/4 teaspoon vegetable oil
Remove the bars from the pan once cooled. Cut into squares and enjoy immediately. Store in an airtight container for up to four days or freeze.