It is a heated butter that take aromatic notes of roasted hazelnut thanks to the casein caramelization. It brings a very interesting aromatic touch to the recipes like the financier cake.
Unsalted Gourmet Butter
Unsalted Tartine et Cuisine
Cut the butter up into small pieces and place in a frying-pan.
Melt the butter over a medium flame, watching carefully.
To keep control of the process, do not turn up the heat too much.
As the butter melts, the casein (the protein in the milk) will form a white scum on the surface.
You can stir the butter to make sure it does not overheat.
As it heats up, the butter soon turns an amber colour and gives off a hazelnut fragrance (this is due to the casein caramelising).
You will know when the butter is done just right when it begins to give off a toasted-hazelnut fragrance.
Immediately add a small piece of fresh butter to slow down the cooking process, then pour the beurre noisette into another container to stop the process completely (otherwise the casein will take on an undesirable black colour).