Line an 8X8 (20.3x20.3 cm) square baking pan with parchment paper. Set aside.
Add the marshmallows to a large bowl, and top with slices of butter.
Heat in the microwave in 30-second intervals until the butter is melted and the marshmallows are puffy.
Stir well to combine the butter and marshmallows, then add in the vanilla extract and stir again.
Pour in the cereal and fold it in with a silicone spatula until completely coated in the marshmallow mixture.
Spread the cereal treat mixture into the prepared baking pan, spreading it out to the edges and into an even layer.
Allow to set at room temperature for at least 1 hour.
Remove the treats from the baking pan and slice into 9 squares.
Notes
Storing: Rice Krispie treats can be stored at room temperature in an airtight container for up to 3 days. For the best results, store the treats between layers of parchment paper to prevent sticking. Treats can be stored in the refrigerator, but will toughen quite a bit, so room temperature is recommended.Freezing: These can be frozen, although the texture changes after freezing. Wrap each treat individually in plastic wrap, then place them in a large freezer bag and label. Freeze for up to 2 months, and thaw the treat before serving.Stovetop Method:
Melt the butter and marshmallows together over medium heat in a very large pot, stirring often.
Remove from the heat and stir in the vanilla extract.
Pour in the rice cereal and fold it in with a silicone spatula until completely coated in the marshmallow mixture.
Spread the cereal treat mixture into the prepared baking pan, spreading it out to the edges and into an even layer.
Allow to set at room temperature for at least 1 hour.
Remove the treats from the baking pan and slice into 9 squares.