Sauce – Mustard Flavor disappears

I came across this article.

It explains where mustard get its heat from:

Mustard seeds come from the mustard plant, a member of the cabbage family. They contain two sulphur compounds, myrosin and sinigrin, as well as an enzyme, myrosinase. When the seeds are broken and water is added, the enzyme breaks down the sulphur compounds. The result is the sharp tasting oil that gives mustard its pungency, and helps explain why the name mustard comes from the Latin words mustum (much) and ardens (burning).

In addition, it recommends adding some flour to tone down its heat.

In the case of prepared mustards, the reaction is toned down by using additives such as flour.

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.