Pour Rice Krispies cereal in a large mixing bowl and set aside.

Reserve 1 cup of the mini marshmallows and set a side to add later as an mix in.

In a large pot, melt the butter over medium heat. Keep cooking, stirring often, until it foams, turns golden, and smells nutty — about 5–7 minutes. Reserve 1 tablespoon if you plan to make the brown butter glaze.

Lower the heat and stir in the mini marshmallows (make sure 1 cup is set aside). Mix until smooth and fully melted. About 5 minutes. Add vanilla and kosher salt, stir till combined and remove from heat.

Pour melted marshmallows over the Rice Krispies cereal until coated. Gently stir in the reserved 1 cup of mini marshmallows for gooey bites.

Grease or line a 9x13-inch pan with parchment or an 8x8 pan for thick and tall bakery style. Lightly press the mixture into the pan with a buttered spatula or the back of the butter wrappers. Don’t pack too tightly — this keeps the treats soft.

Let cool for at least 30 minutes, then cut into squares. Finish with a sprinkle of flaky sea salt if desired.

Whisk together the reserved 1 TBSP brown butter, powdered sugar, vanilla, and 1–2 TBSP milk until smooth. Drizzle over cooled treats.