Melted over medium heat and cooked another 3-5 minutes once it foams up, butter darkens and turns into nutty-smelling brown butter (beurre noisette). Timing can shift by a minute or two depending on the amount of butter and how thick the pan is.
What happens during this transformation?
As butter melts, the water in it evaporates first and the fat starts to foam; once the foam settles, the milk solids in the butter — casein and lactose — start browning at the bottom of the pan. Those browned solids are what give it the characteristic nutty smell and taste.
Why is timing the right moment tricky?
The milk solids can go from light brown to dark brown within seconds, so the butter needs constant watching at this stage, and the pan should come off the heat to check if needed. Once the colour turns from dark brown to black, the butter has burned and the flavour turns bitter beyond saving.
A light-coloured pan makes it much easier to see the colour change clearly; dark or cast-iron pans make it harder to judge, so checking the bottom with a spoon now and then helps.
Why cool it down right away?
Once it reaches the desired brown, the pan should come off the heat immediately and the butter poured into a cold container; butter left in the hot pan keeps cooking from residual heat and can burn within seconds. Scrape the darker solids at the bottom into the container too when pouring — most of the flavour lives there.
How does timing change with a larger amount?
Melting a larger amount of butter, 250 grams or more, can stretch total time to 6-8 minutes, since evaporating the water and settling the foam both take longer. With smaller amounts, 50-100 grams, it can take under 3 minutes, so smaller batches need more frequent checking.
What is it used for?
Brown butter, combined with sage, can be drizzled over mantı or ravioli, and swapped in for plain melted butter in cakes and cookie dough for a clear nutty note. For sweet recipes, let it cool to room temperature before using.
Salted or unsalted butter?
Unsalted butter is usually preferred for browning, since the salt in salted butter can make it harder to judge the browning point, and controlling the final salt level for the recipe gets trickier. With unsalted butter, salt can be added separately as the recipe calls for.