Line your 9x13 inch pan with parchment paper or spray it generously with cooking spray to prevent sticking.
In a large, heavy-bottomed saucepan, melt the marshmallows over medium-low heat, stirring constantly. You’ll hear a gentle bubbling and see the marshmallows turn shiny and smooth, about 5 minutes.
Add the butter to the melted marshmallows and stir until fully incorporated. The mixture will become glossy and fragrant, about 2 minutes.
Remove the saucepan from heat. Stir in the vanilla extract quickly, then fold in the cereal until evenly coated and shiny, about 1-2 minutes. The mixture should be thick but still sticky.
Transfer the mixture into your prepared pan. Use a spatula or wax paper to press it down firmly and evenly, creating a smooth, shiny surface. You should hear a slight crackle as you press.
Let the bars set at room temperature for at least 30 minutes or refrigerate for 20-30 minutes until firm. This helps them hold shape when sliced.
Once set, lift the entire block from the pan using the parchment paper edges. Use a sharp knife to cut into squares or rectangles, wiping the knife clean between cuts for neat slices.
If desired, melt some chocolate and drizzle over the bars for a glossy finish. Let the chocolate set for about 10 minutes before serving.
