The process of browning butter goes through a few stages. It will first melt all the butter into liquid. Then, as the moisture in the fat evaporates, it starts to foam. This is normal and the only thing you need to do is keep the heat low and make sure it does not overflow. As you continue, the foam will disappear and you will start to see the milk solids separate as they gently cook and start to brown. The smell during this process also starts to change from a wonderful buttery flavor to a sweet nutty caramel flavor.