Brown butter (beurre noisette) is butter heated until it turns golden brown and develops a nutty aroma. This is crucial for authentic financiers. Heat butter in a light-colored pan so you can see the color change. The butter will foam, then the foam subsides as water evaporates. Continue cooking until it turns golden brown and smells nutty. The milk solids at the bottom should be golden, not black. Cool slightly before adding to batter. This technique adds depth and complexity to baked goods.