Line a 9x13 pan with foil and grease with butter or cooking spray.
In a large pot on medium low, melt butter. Add miniature marshmallows and stir until marshmallows are melted.
Remove from heat. Add vanilla and stir well.
Add cereal and mix until coated. If using additional marshmallows*, add and stir in.
Fold mixture into the pan and press down evenly (using the back of a greased spoon or measuring cup if needed).
Cool. Using foil (or parchment), remove squares from the pan and cut.
Notes
*if using a 13-14 ounce package of mini marshmallows, remove 1 ½ cups. These can be used for addins
Use a 9x9 pan for taller squares
Store squares at room temperature in a covered container for up to 4 days.
Rice Krispie Squares can be frozen! Wrap squares in parchment paper and place in a freezer bag for up to 3 months.
Microwave Method:
Line a 9x13 pan with foil and grease with butter or cooking spray.
In a large microwave safe bowl, heat butter on high for 45 seconds, or until melted. Add miniature marshmallows and stir.
Microwave another 45 seconds and stir. Microwave an additional 45 seconds and stir. If marshmallows are not fully melted, microwave in 10 second increments, stirring after each 10 seconds.
Once melted, proceed with recipe starting at step 4 above.