In a large pot over low heat, melt the butter. Add the marshmallows and stir continuously until completely melted and smooth.
Remove from heat and stir in the vanilla extract. If using multiple colors, divide the mixture into separate bowls and add food coloring to each.
Gently fold in the Rice Krispies cereal, stirring until evenly coated with the marshmallow mixture.
Transfer the mixture to a greased 9x13-inch pan. Lightly press down with a greased spatula to spread evenly, being careful not to pack them too tightly.
If desired, sprinkle with edible glitter or sprinkles before the mixture sets.
Let cool for about 30 minutes, then cut into squares and serve.