Place the butter in your skillet or saucepan over medium heat. If using a solid block of butter, you can drop it in the pan just like that or slice up the butter into small pieces.
Stir occasionally as it melts completely (about 1 - 3 minutes). It will look smooth and yellow.
Once melted, it will start to foam, sizzle, and bubble as the water evaporates. Keep stirring constantly (or swirling the pan) to distribute the heat evenly. The foam will rise, then subside a bit.
After 4-6 more minutes (the entire process takes approximately 5 - 10 minutes), you'll see tiny golden - brown specks (that's the milk solids) forming at the bottom. The butter will start turning amber/golden tan.
When the specks are deep golden-brown and the aroma is nutty (like a toasted hazelnut) pull the pan off the stove. (Pay attention so your butter doesn't get dark brown-there's only seconds before it burns). Pour the butter (including the browned bits) into a heatproof bowl or jar to stop cooking. I like using clear jars.
Let it cool slightly before using (it solidifies as it cools). If not using right away, store in the fridge (up to 2 weeks) or freezer (2-3 months).