In a large saucepan melt butter over low heat. Add marshmallows and green food coloring dye and stir until completely melted. Remove from heat.
Add Rice Krispies Cereal. Stir until well coated.
Using butter spatula or wax paper evenly press mixture into 13 x 9 x 2-inch coated with cooking spray. Cool.
Spray shamrock cookie cutter with cooking spray. Cut out Rice Krispies Treats with shamrock cookie cutter.