Spray a 9 x 13 x 2 baking pan with cooking spray, or brush with butter. Set aside.
Melt butter over LOW heat in a large saucepan or Dutch oven.
Add the marshmallows, toss to coat, and continue stirring until completely melted. Remove from heat.
Add 4 to 5 drops green coloring and the vanilla extract, stir to completely combine. Add the crispy rice cereal and stir until well coated.
Pour the mixture into the prepared baking pan and gently to spread out evenly. Tip - rub butter in between your hands so they don't stick to the marshmallow treats.
Sprinkle with colored sanding sugar and allow to cool completely for an hour. Cut into 2-inch squares and add a jumbo red heart before serving. I used Cake Drip, but a tube of icing works as well.
Notes
For best results, melt the butter and marshmallows on low heat!
Use the freshest ingredients possible.
A 7-ounce jar of marshmallow creme can be substituted for the regular marshmallows.
Do not use margarine or diet/reduced calorie butter. They contain water which will make your treats soggy, and they will not have the same homemade flavor.
These holiday rice krispie treats are best served the day they are made, but they will last in an air-tight container for 2 to 3 days.
To make shapes: spread mixture out on a prepared baking sheet to about 1-inch thick. Allow to cool, then use cookie cutter to cut out hearts, trees, shamrocks, circles, etc.
Substitute pink or red for Valentine's Day, remove the heart for St. Patrick's Day. You can use any color of gel coloring and sugar based on your party theme.