Prep the pan: Line an 8x8 or 9x9-inch pan with parchment, leaving overhang. Lightly butter or spray.

Measure everything first: Have cereal in a large bowl. Reserve 1 cup mini marshmallows for folding at the end.

Brown the butter: In a medium saucepan over medium heat, melt the butter. Cook, stirring, until it foams, then turns golden with brown bits on the bottom and smells nutty, 4–6 minutes. Watch closely so it doesn’t burn.

Melt the marshmallows: Reduce heat to low. Add 10 oz mini marshmallows, a pinch of salt, and vanilla. Stir until smooth. For extra chew, stir in corn syrup or honey now.

Combine with cereal: Pour the marshmallow mixture over the cereal. Stir quickly with a buttered spatula until mostly coated, then fold in the reserved 1 cup marshmallows for pockets of goo.

Press gently: Transfer to the pan. Lightly butter your hands or use parchment to press the mixture into an even layer. Do not pack tightly or they’ll be dense.

Make the peanut butter drizzle: In a small bowl, microwave peanut butter with 1–2 tsp oil in 10–15 second bursts until fluid. Stir until smooth and pourable.

Finish: Drizzle peanut butter over the top. Sprinkle with flaky sea salt. Let set at room temperature 30–45 minutes.

Slice: Use the parchment overhang to lift the slab. Cut into 9–16 squares. Wipe the knife between cuts for clean edges.