Clean, then coat inside of plastic eggs with cooking spray. Set aside.
In large saucepan, melt butter over low heat. Add marshmallows and stir until completely melted. Remove from heat.
Add Kellogg's® Rice Krispies® cereal. Stir until well coated.
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. Remove from molds. Place on wax paper. Cool slightly.
Place about 6 M&M'S® Brand Chocolate Candies in one half of each egg. Gently press two halves of each egg together until they stick. Cool completely.
Decorate with frosting and additional M&M'S® Brand Chocolate Candies (if desired). Best if served the same day. Serving size is 1 egg.