Melt butter in saucepan over low heat. Add marshmallows and stir till melted. Cook 3 minutes, stirring constantly. Remove from heat, add Rice Krispies and stir till all are coated. Using buttered spatula, press evenly into buttered 13x9x2" pan. Cool. Cut into 2" squares.
VARIATIONS: add 1 cup raisins add 1 cup peanuts add 1/4 cup peanut butter to marshmallows melt 2 squares chocolate with marshmallows for Christmas: add green food colouring (if desired), shape into "trees" or press into buttered ring or small Bundt mold. Decorate with red cinnamon candies (for tree) or spearmint leaves and jelly berries for ring mold (resembles a wreath)