Prep Pan: Grease a 9x13-inch baking dish.
Melt Base: Place butter and marshmallows in a large microwave-safe bowl. Microwave on HIGH for 1 minute. Stir. Microwave 30–45 seconds more until completely puffed.
Mix Flavors: Immediately stir peanut butter and vanilla into hot marshmallows until smooth.
Fold Cereal: Gently stir in Cheerios until evenly coated.
Press: Press mixture into the prepared pan using a greased spatula.
Cool: Let sit 45 minutes at room temperature before slicing.