Prepare a 9-inch square baking pan by greasing it well with butter.
In a large pan on the stove, melt the butter over medium heat.
Reduce the heat to medium-low; add all of the marshmallows EXCEPT for 1 cup.
Stir constantly, until the marshmallows are melted.
Remove from the heat, and stir in the vanilla.
Add the rice crispy cereal and the remaining one cup of mini mallows, and stir to combine well, making sure there are no dry pockets of cereal.
Add the cereal to the prepared pan, and use a spatula to spread it out to an even layer, taking care not to press or smash it down. If a spatula isn't cutting it, get your hands lightly damp, and gently press out the cereal. A buttered bottom of a measuring cup also works well.
Let the rice crispy treats cool for 30 minutes before serving.
Notes
Keys to avoiding hard rice crispy treats: Cook your mallows at a low temp, use fresh marshmallows, don't press things too hard into the pan, and use lots of butter (I use more than double what the "on the box" recipe calls for, and it's delightful).
Grease your hands to help push the mixture into the pan evenly.
Fruity Pebbles make a great substitute.
For special occasions you can add mix-ins like chocolate chips, mini chocolate candies, sprinkles, etc.
They will be easier to cut if you wait until the mixture is completely cool.