Line an 8 x 8-inch square tin with parchment, and spray generously with nonstick spray.

Add the butter to a large, heavy-bottomed saucepan or stock pot. Melt the butter.

Add the marshmallows, stirring continually until completely melted.

Remove from heat. Stir in vanilla extract and fine sea salt.

Add the cereal and stir until fully combined, with no dry cereal remaining.

Pour cereal mixture onto prepared tin.

With damp fingertips, pat down gently until evenly distributed. (Do not press too firmly, or the treats will be difficult to chew!)

Set aside. Cool at room temperature, at least 1 hour.

Lifting by the parchment overhang, transfer cooled slab to a cutting board.

With a sharp, well-oiled chef’s knife, slice into squares.

Serve immediately.

Arrange leftover treats in an airtight container, layered between sheets of parchment. Store at room temperature for up to 3 days.