Melt the butter over a medium heat. Let it foam up well, then turn the heat down slightly. Keep cooking until the butter browns and smells distinctly nutty - stir as you go.
Chill the butter until it has set, then let it come to room temperature.You can make the brown butter the day before and keep it in the fridge. The next day, simply take it out of the fridge and leave it until it is soft and at room temperature.
Put the butter in a bowl and whip it in a stand mixer or with a whisk until pale. This takes about 5 minutes.
Add the salt and whip for another couple of minutes, until the butter is light and airy.
Serve straight away – with good freshly baked bread.
The butter can be made ahead and kept in the fridge – but it will naturally firm up. Take it out about 1 hour before serving so it becomes soft again – it keeps its airy texture well.