Line your 9x13 inch pan with parchment paper to keep the bars from sticking.
Combine marshmallows and butter in a large heatproof bowl. Microwave on high for about 1 minute until the mixture is bubbling and smooth when stirred.
Stir the melted mixture until glossy and fully combined, then add vanilla extract and mix again to incorporate the aroma.
Gently fold in the cereal or crushed graham crackers, mixing just until evenly coated but still slightly crunchy.
Pour the mixture into your prepared pan and press it down firmly with a spatula or the back of a spoon, creating an even, compact layer.
Let the bars rest at room temperature for about 10-15 minutes to set slightly, then transfer to the fridge and chill for at least 1 hour until firm.
Once chilled and firm, remove from the fridge and lift the bars out of the pan using the parchment paper. Slice into squares with a sharp knife, wiping it clean between cuts for neat edges.
If desired, melt your chocolate and drizzle over the bars for a decadent finish. Sprinkle with a pinch of sea salt for contrast.
Serve immediately or store in an airtight container at room temperature for up to 3 days or in the fridge for up to a week.
