What is brown butter?

Butter cooked past the melting point until the water content evaporates and the milk solids toast on the bottom of the pan, turning golden brown and developing nutty, caramel-edged flavors. Called beurre noisette (hazelnut butter) in French. Cooked at medium heat with frequent swirling, usually 8-12 minutes total: the butter melts, foams, quiets, then the milk solids hit the bottom and start browning — pull from heat the moment you smell nuttiness. Browning evaporates ~15-20% of the butter weight as water, so browned butter weighs less than the raw amount you started with. Used in brownies, financiers, and chewy chocolate-chip cookies for depth that plain melted butter cannot produce. ⚠️ Pour hot brown butter into metal or ceramic — glass containers can shatter, especially in an ice bath cool-down.