This is how to fix a broken sauce like mayonnaise, aioli, Béarnaise or Hollandaise.
When you are making a sauce and the egg yolk and oil don’t emulsify into a creamy sauce it is ‘broken’.
You can repair it. It isn’t difficult but it takes a bit of time and patience.
There are a couple of ways to re-emulsify the sauce.
- Take a fresh egg yolk in a new clean bowl. Whisk the egg yolk well and then while continuing to whisk manually, add the broken sauce a drop or very small splash at a time. You should see each drop get incorporated in a creamy solid sauce. If you try to go too quickly you may see tiny fat droplets forming around the outer edge of the sauce. If this happens stop adding fat and add a few drops of liquid, whisking. You should see the sauce ‘tighten up’ and become consistently creamy again. Then you can resume adding the broken sauce.
- The other approach is to start with about 1/ 2 teaspoon of water and slowly whisk in the broken sauce bit by bit according to the instructions above. Start with just a few drops of the broken sauce and whisk until it is well, well integrated. After a couple of additions you should see the new sauce getting more opaque. With each addition it will appear creamier and thicker. If this doesn’t happen after you have added a Tablespoon of the broken sauce or more then you have to start over. If the sauce doesn’t emulsify right from the beginning it will not correct itself later. So watch the colour and texture carefully.
Note- If you sauce is ‘broken’ because you have curdled the eggs with too much heat you cannot re-emulsify it. Your only choice here really is to start over completely. If you want to try to save the butter portion you can try straining out the egg solids. Start whisking new egg yolks according to your recipe and gradually whisk in the strained oil component.
Water, whisk, new vessel needed to repair broken sauce. 
Stat with about 1/2 tsp of water in the new vessel. 
Add broken sauce a few drops at a time while whisking vigorously. 
Sauce should start to thicken and colour turn opaque after a few additions. 
Note that the sauce now coats the whisk instead of running off. 
Final consistency should be as thick as you expect. Note how it stays on the knife blade.
How to Fix a Broken Sauce
Dressing didn't emulsify? Sauce separated?
Equipment
- small vessel
- whisk
Materials
- One broken sauce
- 1/2 tsp water or
- 1 egg yolk
Instructions
- Before you start – if your sauce is broken because you cooked the egg you cannot fix it. You will need to start over.
- If it is broken because it didn't emulsify (get creamy) while you were adding the oil then follow these steps to integrate it.
- Start with EITHER one whisked egg yolk OR 1/2 tsp water in a small vessel just large enough to hold your sauce and some room to allow for vigorous whisking.
- Start by adding a few DROPS of the broken sauce to the whisked egg yolk or the water. Whisk vigorously. Add a couple more DROPS. Try to whisk continuously. It may be awkward to hold the cup and the whisk and add the broken sauce. Whisk vigorously. You should see from the first few drops added that the sauce is changing from rather transparent broken texture of the broken sauce to a more opaque creamier sauce. It is subtle at first and becomes more obvious as you re-integrate more sauce.
- If the sauce is not getting thicker and more opaque after say a tablespoon or so of the broken sauce has been added then start over and whisk more vigorously as you are adding the broken sauce. If it doesn't integrate in the beginning it will not correct itself later.
- Keep whisking and adding the sauce slowly – like a 1/2 tsp at a time. Ensure that each addition has been thoroughly incorporated and takes on a creamy texture.
- Continue until you have re-incorporated all the broken sauce.
