Line your 9x13 inch baking dish with parchment paper, leaving some overhang for easy removal.
In a large saucepan, melt the butter over low heat until it’s silky and bubbling gently, filling your kitchen with a warm, buttery aroma.
Add the marshmallows to the melted butter, stirring constantly with the spatula. Watch as they melt into a sticky, glossy mixture, releasing a sweet, toasted scent, about 3-4 minutes.
Remove the saucepan from heat and quickly stir in the vanilla extract, mixing until fragrant and well combined.
Immediately add the crispy rice cereal to the marshmallow mixture, folding gently with the spatula until the cereal is evenly coated and the mixture is shiny and sticky.
Pour the mixture into your prepared dish and use the spatula or wax paper to press it evenly into the pan, smoothing the top without compressing too hard.
Let the bars sit at room temperature for about 30 minutes, allowing them to set and develop a glossy, golden finish. For quicker setting, refrigerate for 20-25 minutes.
Once set, lift the bars out of the dish using the parchment overhang and transfer to a cutting board. Use a sharp knife to cut into squares, wiping the blade clean between cuts for neat, straight edges.
Serve immediately for the best chewy texture or store in an airtight container at room temperature for up to three days. For cleaner slices, refrigerate for 10-15 minutes before serving.
