Melt butter in a medium saucepan with tall sides (a few inches or so depending on the amount of butter you're browning) over medium heat.
It will foam at first and spurt, so be careful! Don't stand too close to the saucepan.
It will melt and will start to bubble and make a furious noise (this will be louder depending on the amount of water the butter has).
Then, the bubbles will become smaller and it will gradually stop chirping (really, it's a chirping noise, you'll see) and will foam quite a bit (images below).
Be careful as it will start browning underneath the foam. You will smell a nutty aroma.
It's very important that you split the foam with the spoon or spatula you're using and see that it doesn’t burn. You want it to be an amber color. Similar to caramel, it goes from amber to burned in a short moment, so be careful.
Take it out and put it in a shallow pan so it stops cooking (image below, right).
The shade goes from light to dark in seconds, just like that (image below). The flavor deepens as it colors, but be aware that, just like it happens with caramel, it goes from wonderfully dark-colored and deep flavored to burned and useless.
Allow to cool and refrigerate until cold. Read the recipe carefully, because usually butter is used at room t°, so it doesn’t need to solidify completely.