Lightly spray a 9x9-inch baking pan generously with cooking spray or lined with greased parchment paper so it comes up the sides and set aside. Using parchment reduces the risk of the treats sticking in the pan if you don't spray it enough.
Cut the butter into four tablespoons and add them to a microwave-safe mixing bowl with the mini marshmallows. Microwave for 90 seconds.
4 tablespoons unsalted butter, 12 ounces mini marshmallows
Stir until the marshmallow and butter mixture is smooth, then add the vanilla extract, food coloring, and salt. Stir until the mixture is evenly green in color.
1 teaspoon green food coloring, 1 teaspoon vanilla extract, ¼ teaspoon salt
Add the rice krispies cereal to the bowl and make sure to evenly coat the cereal with the marshmallow mixture.
5 cups Rice Krispies cereal
Use a greased rubber spatula to transfer the Rice Krispie mixture to the prepared baking pan. Gently press the Rice Krispies into the pan, making sure the top layer is as even as possible. I like to spray my hands with nonstick spray or use another piece of greased parchment paper to press the mixture.
Let the Rice Krispies cool and firm up in the pan for 1 to 3 hours.