Beurre Monté
Beurre Monté is a butter emulsion that resists separating at higher temperatures than butter alone. Which is itself an emulsion of course. Whilst butter begins to break down around 70°C (158°F), beurre monté will stay emulsified up to 82-88°C (180-190°F).
Formed by whisking butter into small amounts (1-4 tablespoons) of near boiling water, it can be kept warm (say in a bain marie) for hours before use covered with cling film or a tight-fitting lid.

You can use beurre monté:

Beurre Monté
butter emulsion
veg ingredient
Makes as much as you need

Bring a small amount of water to a simmer in a heavy saucepan. Reduce the heat to low and gradually whisk in the chilled butter one or two chunks at a time to emulsify into a rich and glossy sauce, keeping the temperature below 88°C(190°F) to prevent separation, until you have the quantity you need.
Keep it warm until required.
Once the emulsion is started you can add in just as much butter as you need, even pounds.