You will need an Immersion Blender (aka stick blender) and a 30oz Stainless Steel Double Wall Vacuum Insulated Mug.
Make sure the head of your stick blender fits all the way down into the bottom of mug.
Immersion Blender example
Insulated Mug example
5 Egg yolks
4 T Water
3 T Lemon juice
1/4 t table salt
1 1/2 sticks salted butter - not melted - cut into chunks
Heat your Sous Vide to 147F.
Put everything except the butter into a vacuum sealer bag (like FoodSaver) or a gallon-size FREEZER ziploc.
Squeeze the bag to break the yolks and mix with the other ingredients.
Add the butter chunks.
If using a FoodSaver bag: carefully seal the bag, taking care not to suck the liquid ingredients into the sealing appliance.
If using a Ziploc: leave the bag partially unzipped so air can escape, lower the bag into the water bath to force out the air in the bag, then zip it.
Clip the bag to the side of your Sous Vide vessel. Cook for 1 - 2 hours.
It will look incredibly icky at this point, but don't despair.
Pour it into the insulated stainless steel cup and blend it with a stick blender.
Put the lid on the cup, and the sauce will stay warm for at least an hour.
Make Ahead
After the initial cook at 147F, refrigerate the bag. Yes, it looks like shit at this point.
This can stay refrigerated for a day or two.
Then, while you're sous vide cooking your steaks, clip the bag to the inside of your Sous Vide vessel for the last hour or so.
It doesn't matter what temperature you're using for your steaks.
Pour into an insulated stainless steel cup and blend with a stick blender.
Put the lid on the cup, and the sauce will stay warm for at least an hour.
Leftovers
If you have any leftover sauce, refrigerate it in a non-insulated container.
The leftovers will be fluffy and spreadable right out of the fridge the next day.
The Johnson Family Cookbook