In a small saucepan, simmer cider vinegar and dashi (if using) over high heat until reduced to about 1 tablespoon (3-5 minutes). If not using dashi, just simmer the vinegar.

Transfer the vinegar-dashi concentrate to a small mixing bowl. Secure the bowl with a damp dish towel ring.

Add Dijon mustard and egg yolk to the bowl and whisk to combine.

While constantly whisking, slowly drizzle canola oil down the side of the bowl into the yolk mixture to emulsify and thicken.

Once thickened, drizzle the oil more quickly while continuing to whisk.

Incorporate all the oil into the mixture.

Mix in sugar and kosher salt.

Add a little dashi or water to thin the mayonnaise for easy squeezing from a bottle.

Transfer the mixture to a squeeze bottle.

Refrigerate the mayonnaise. It will thicken slightly. Use within 1 week.

If the mixture separates, fix it by whisking the broken mixture into a fresh egg yolk as if it were the oil.