Butter a 9x9-inch square baking dish (for extra thick bakery-style treats) or an 8x8-inch dish, or line it with parchment paper.

In a large Dutch oven or non-stick pot over medium-low heat, melt the unsalted butter. Continue cooking, swirling the pan frequently, until the butter foams, turns golden amber, and releases a nutty aroma (about 3 to 4 minutes).

Reduce heat to low. Add 12 ounces of the mini marshmallows to the browned butter. Stir continuously with a silicone spatula until the marshmallows are about 90% melted.

Remove the pot from heat immediately. Stir in the pure vanilla extract and flaky sea salt until combined.

Add the crisped rice cereal and the remaining 4 ounces of unmelted mini marshmallows to the warm mixture. Fold gently until the cereal is evenly coated and marshmallow pockets are visible throughout.

Transfer the warm mixture into the prepared baking dish. Using buttered fingertips or a sheet of wax paper, gently guide the mixture into an even layer. Do not compress or press down firmly.

Allow the treats to set at room temperature for 30 minutes before lifting them from the pan and slicing into 16 thick squares.