Try this:
Start melting butter over medium heat. Use a pan with a light-colored bottom so you can keep track of the color. Swirl the pan occasionally to be sure the butter is cooking evenly.
As the butter melts, it will begin to foam. The color will progress from lemony-yellow to golden-tan to, finally, a toasty-brown. Once you smell that nutty aroma, take the pan off the heat and transfer the browned butter into a heat-proof bowl to cool.
The milk solids will cook faster and you'll see them settle on the bottom of the pan like the dregs in a bottle of wine. When you transfer the brown butter to a new container, try to leave as much of this sediment in the pan as possible. You can also strain the butter through a fine meshed strainer or cheese cloth to remove all the particles. Pour over the freshly popped popcorn and sprinkle very lightly with extra fine sea-salt..." (From an online site, but we've used for ages).