Will stay fresh for weeks, because the casein and whey, which are subject to bacterial change, have been removed. Also has a higher burning point than butter and is a good choice for delicate sauteeing without browning the butter.
1/2 pound (2 sticks) butter
Preheat oven to 300 degrees. Cut butter into small pieces and place in heatproof bowl in oven. Leave until melted.
Remove from oven. Skim off foam (casein) and discard.
Let stand for a few minutes to allow milk solids to settle. Skim clear yellow liquid (the clarified butter) off the milky residue (whey) left on the bottom and store in a covered jar in the fridge. Discard residue.