Why didn't my mustard sauce emulsify? I was making mustard croutons. I whisked together 2 parts warm melted butter, 2 parts olive oil, 1 part Dijon mustard, 1 part brown mustard (with whole seeds), 1 part sherry vinegar and table salt.

Since I had so much mustard I didn't think it would be necessary to do anything gradually (mustard being a great emulsifier), so I just dumped it all together and started whisking. Is that likely to have been the problem? Could it have been the warmth of the butter (not quite hot)?

Is there such a thing as too much emulsifier? The sauce never did emulsify by whisking, it just stubbornly stayed completely separated, albeit with ever smaller droplets of mustard/vinegar.

It probably wouldn't have mattered much since I was just planning to toss bread chunks in it anyway, to bake for croutons, but it bugged me so I whipped out the food processor. After processing the sauce looked emulsified, but it separated within perhaps 2 minutes. The croutons turned out great anyway, but for future reference I'd still like to know why I never did achieve an emulsion. I've made mayonnaise pretty much the same way, and it always turns out. What gives?
You really need to add the oil slowly at first, especially if whisking. You are trying to disperse the fat into tiny droplets suspended in the water phase (the water from the butter and the vinegar). You can't whisk fast or hard enough, especially by hand, to break up 3.7 parts fat in 1.3 parts water when all the fat is trying to combine into a single mass floating on the water, no matter how much mustard you have.

I would have whisked the mustard into the vinegar, then slowly added the butter, then the oil.