Beurre noisette, or brown butter, is butter cooked past melting until its milk solids toast to a golden brown, giving it a nutty aroma and flavor -- the French name translates to "hazelnut butter." It's made by heating unsalted butter over medium heat until it foams, then watching closely as the solids sink and caramelize, taking it off the heat as soon as it turns golden-brown to avoid burning.
It's used to finish sauces like beurre blanc or hollandaise, drizzled over roasted vegetables, fish, or meat, and is central to dishes like sole meunière, where the fish is sautéed and finished with the browned butter. In baking, it flavors financiers, madeleines, and shortbread, and can be folded into pastry cream, frosting, or ice cream base. Herbs, spices, or citrus zest are common additions during the browning.