In a medium saucepan, cook butter over medium heat until melted and bubbling. Reduce heat and simmer gently, stirring occasionally, until only a light froth remains on top and milk solids at bottom of pan just start to turn golden, 25 to 30 minutes. Let cool about 10 minutes (milk solids may turn darker.)
Skim any foam on top with a spoon. Pour butter through a fine strainer or a few thicknesses of cheesecloth.
MAKE AHEAD Keeps, chilled, up to 1 month, or frozen up to 3 months.