1. Clean, then coat inside of plastic eggs with cooking spray, set aside.
2. In large saucepan, melt butter over low heat. Add marshmallows and stir until well coated. Remove from heat.
3. Add Kellogg's Rice Krispie's cereal, stir until well coated.
4. Using greased hands, firmly press 1/4 cup of Rice Krispies mixture into both halves of each plastic egg. Use fingers to make hollow center in each half.
5. Remove from mold. Place on wax paper. Cool slightly. Place about 6 M&M's candies in one half of each egg. Gently press two halves of each egg together until they stick. Cool completely.
6. Decorate with frosting and additional M&M's candies (if desired).