How can I prevent bacon mayonnaise from splitting when above fridge temp?

I think the problem is actually not the heating, but the refrigeration!

Consider: bacon fat solidifies easily and thickly, and does so even at room temperature. When you throw it in the fridge, the micro-droplets of bacon fat will turn to solids and clump together. These solid droplets are frozen in place when solid, but when you thaw it, they melt and reveal how wrecked your emulsion is. Now, you could add additional emulsifying agents (lecithin, extra yolks) but that's not really going to solve the problem of refrigeration.

The solution is to break up the bacon fat as it melts, and re-establish the emulsion before it can break. To do this, you have to whisk constantly as you gently heat the mayonnaise, generally in a warm water bath.

How to execute this in a restaurant setting:

Prep a big batch of bacon-mayo and throw it in the fridge. Just before service, warm some water in a pot and throw a cup of the mayo in a small bain marie or metal 6th hotel pan. Immerse the bain/pan in the pot, and whisk it as it melts. Hold the mayonnaise for service in water warm enough to melt bacon fat, and DISCARD THE WARM MAYONNAISE EVERY TWO HOURS AND THAW A FRESH BATCH. Make sure no cooks get lazy about that -- it's a food safety problem. Ideally, you should be using pasteurized eggs to reduce the risk of salmonella. Basically, you're treating your mayonnaise like a Hollandaise or Bearnaise.

I've found that the thawing trick works fairly well for home hollandaise (which most people say shouldn't refrigerate), and which does the same thing if heated fast. It should apply to easily-broken mayonnaise too.

This is purely a guess, but I'm thinking that a little xanthan gum, say 0.5% by weight, might stabilize it. I'd shear it in with the yolk/mustard/vinegar, let it hydrate for ten minutes, make the mayo, then pass it through a tamis to make sure there aren't any gritty bits. If you try it, let me know if it helps.

I've had difficulties with breaking emulsions in Mayo before, and it can happen for many reasons, even with enough egg present. It might be worth trying to reset the emulsion:

Lightly beat 1 egg yolk in a bowl, beat in a bit of the mayo (warmed slightly to work with), and transfer to a blender (easier than using a whisk; if you have hulk forearms, you can whisk away), and pulse to emulsify (a second or two). Continue adding more mayo to the mix and pulsing for a few seconds until all the mayo is incorporated. If using the blender, be careful not to overwork the mayo, as this may also break the emulsion. If you do overwork it, you can always just start the process again with another yolk.

I've had home-made mayos break on me for no particular reason, and rebuilding the emulsion with a fresh yolk fixed the problem.

A commercial emulsifier like soy lecithin might be easier (and more effective). It is often added to emulsions (salad dressings, ice cream, store-bought mayo, etc.) to stabilize them when shipping and storing in a wide range or temperatures. Lecithin doesn't have any real flavor and doesn't modify the texture much other than holding the emulsion together.

From your ingredient list, you don't have enough water to keep the fat molecules separated. I normally include some lemon juice and water (as well as the vinegar), adding half at the start and half once half the fat is emulsified.

To correct, you should be able to:

• Start a new emulsion with another yolk and a tablespoon of water
• Slowly incorporate the baconated mayo back into the new emulsion
• Add another few teaspoons of water once half the bacon mayo is re-emulsified

You may have to continue to add water if the result feels thick (up to a few more tablespoons).

Not speaking from direct experience, but from what I've heard about this, you're going to get splitting with the bacon fat - possibly because there is more saturated fat in there. You may just have to increase your ratio of vegetable oil to bacon fat until it's more stable.