Monter au beurre: gloss without thickening

Cold butter whisked into a hot sauce off the heat. It is an emulsion, which is why it has to be cold, and why it cannot boil afterwards.

1. Take the pan off the heat and wait ten seconds. Butter added to a boiling sauce melts into fat and floats; the shine you wanted turns into an oil slick.
2. Add the butter cold and in cubes, one at a time, swirling the pan rather than stirring. Roughly 40 g to half a litre of sauce.
3. Serve within a few minutes. A mounted sauce held on a warm stove splits; it cannot be remade in the pan.