Line a 9x13-inch pan with foil and spray with nonstick cooking spray.

Melt the butter in a large pot over medium-low heat. Add the marshmallows and stir until melted. Remove from heat and stir in vanilla extract.

Add cereal and stir to coat with the marshmallow mixture. Pour into the prepared pan. Press firmly to compact. Tip: spray wax paper with nonstick cooking spray so you can press without it sticking to your hands.

Let set until firm, then slice into bars. Store in an airtight container for up to 3 days.