In a large, light-colored saucepan over medium heat, melt the butter, swirling constantly. Watch for foam, then golden specks—once the aroma smells nutty (about 4–5 minutes), remove from heat immediately to avoid burning.

Off the heat, whisk in the coarse sea salt and vanilla extract; this amps up the complexity.

Add all the mini marshmallows to the browned butter pan, return to low heat, and stir continuously until smooth—no big clumps, about 2–3 minutes. If you see streaks of white, keep stirring gently until fully melted.

Quickly add the Rice Krispies, gently folding with a silicone spatula so every crisp is coated in that gooey, browned butter mixture. Work swiftly but gently—overmixing can crush the cereal.

Transfer mixture into a parchment-lined 9×13-inch baking dish. Use lightly buttered hands or an oiled spatula to press evenly—press too hard and treats turn brick-like; too soft and they fall apart. Aim for a light, even layer.

Sprinkle chocolate chips or nuts on top; press them in lightly so they stick. If you prefer them mixed throughout, fold in before pressing.

Let the pan rest at room temperature for 10 minutes, then refrigerate for at least 20 more. Slice into squares or fun shapes using a sharp knife dipped in warm water for clean cuts.