Lightly spray a 9×13-inch pan with nonstick spray and set aside.
In a large pot over low heat, melt the butter and dissolve the salt.
Add the mini marshmallows and stir until completely melted and smooth. Remove from the heat.
Stir in the vanilla extract. Then fold in the Rice Krispies until evenly coated.
Transfer the mixture to the prepared pan. Use lightly buttered or damp hands to gently press the treats into an even layer. Don’t compact them, or the bars will turn dense.
Let cool for 1 hour, then cut into 16 squares.
Add the green frosting to a piping bag fitted with a grass tip (#233). Pipe the frosting over the front of each Rice Krispie treat.
Add the red frosting to a piping bag fitted with a small round tip. Pipe a heart in the center of each treat. Enjoy!