Prepare an 8 X 8-inch pan by lining it with parchment paper or simply rubbing some oil on the bottom of it. (You want to have the pan ready as soon as all of the ingredients are mixed together since it will set quickly.)
Melt 1/3 cup of vegan butter in a medium-sized saucepan over medium-low heat. Add a 10 oz. bag of vegan marshmallows and stir well. (Mini marshmallows will melt faster and easier.)
Continue to heat the butter and marshmallows, stirring frequently until they are melted. (They will take longer than traditional marshmallows to melt.)
Once they are melted they will be a big sticky blob. At this time stir in 1 teaspoon of vanilla extract until you can no longer see the vanilla, then stir in 4 1/2 cups of puffed rice cereal.
Work quickly to mix it evenly, then pour it into your prepared dish.
Put a little cooking oil on your hands and press it down evenly in the pan. (At this time you can add some sprinkles to make them more festive if you wish.)
Allow cooling for at least 30 minutes before cutting into squares.
Notes
The melted marshmallows are very hot and sticky! Don't touch the melted marshmallows and watch children if they are helping you.
If you are used to making these with regular marshmallows, the vegan marshmallows take a little longer to melt.
You can place a lid on the pot in between stirring to help trap in the heat and help melt them faster.
A wooden spoon works best for stirring in the cereal since it gets very thick.
Add sprinkles while they are still warm and sticky if desired.
Be sure to use fresh cereal, stale rice krispies will give you stale rice krispie treats.
Microwave method
Prepare an 8 X 8-inch pan by lining it with parchment paper or simply rubbing some oil on the bottom of it. (You want to have the pan ready as soon as all of the ingredients are mixed together since it will set quickly.)
Add the margarine and vegan marshmallow to a large microwave-safe bowl and microwave on high for 1 minute.
Stir the marshmallows well getting them coated in the melted marshmallow then put it back in the microwave for 30 more seconds.
Keep stirring and reheating the marshmallows in 30-second intervals until the marshmallows have lost their shape and are very soft. This usually takes about 3 minutes total in the microwave.
Stir in 1 teaspoon of vanilla extract and then add 4 1/2 cups of puffed rice cereal.
Mix well and then pour into your prepared dish.
Allow cooling for at least 30 minutes before cutting into squares.