Line your baking pan with parchment paper, leaving overhang for easy removal.
Add marshmallows and butter to a microwave-safe bowl. Microwave in 30-second bursts, stirring after each, until the mixture is smooth, gooey, and fragrant—about 1-2 minutes.
While the marshmallows melt, crush the graham crackers into fine crumbs using a zip-top bag and rolling pin or a food processor, then set aside.
Pour the melted marshmallow mixture into a large bowl and gently fold in the crushed graham crackers until evenly coated and sticky—this helps the mixture come together nicely.
Transfer the sticky mixture into your prepared pan, spreading evenly with a spatula. Press down gently to compact the mixture and eliminate air pockets.
Sprinkle a pinch of flaky sea salt over the top for a salty contrast, pressing it lightly into the surface.
Let the bars rest at room temperature for about 30 minutes, or refrigerate for 15-20 minutes until they firm up.
Once set, lift the entire block out using the parchment overhang and transfer to a cutting board.
Use a sharp knife to cut into squares or rectangles, wiping the blade clean between cuts for neat edges.
Optional: Melt dark chocolate chunks and drizzle over the cut bars for extra decadence, or serve as is for a simple treat.
Enjoy your gooey, salty-sweet marshmallow bars immediately or store in an airtight container for up to 3 days for maximum freshness and softness.
