Butter a 9x13" pan and set aside. Measure the rice krispie cereal into a bowl.
In a large glass bowl, melt the butter in the microwave. Add the marshmallows. Continue to cook, stirring every 20-30 seconds, until the marshmallows are melted.
Stir in the vanilla and salt. Add the cereal and quickly stir until completely coated with marshmallows. If adding candy, stir it in as well.
Press the cereal mixture into the prepared pan using wet hands. (Or form them into nests.) I like to press additional candy into the top of the bars.
Cut into squares, or dump out and use Easter shaped cookie cutters.
If desired, decorate the bars or shapes with melted chocolate, sprinkles, or candy.
Notes
Instead of using butter, I like to dip my hands in a glass of water to keep the rice crispy mixture from sticking.
Don't press the cereal mixture too firmly into the pan, or your squares will be hard to bite into.
For thicker bars, you can use a 7x11" pan instead of a 9x13".
You can fill the nests with green frosting before adding the candy, but I think it makes them a little too sweet.