Step 1

In a small saucepan over high heat, bring the cider vinegar and dashi, if using, to a simmer (if you aren't using dashi, just simmer the vinegar). Cook, adjusting the heat to keep it simmering, not boiling, for 3 to 5 minutes, until reduced to about 1 tablespoon.

Step 2

Transfer the vinegar-dashi concentrate to a small bowl. Form a ring with a damp dish towel to rest your bowl on—this will keep it stable. Add the egg yolk and mustard and whisk to combine.

Step 3

Whisking constantly, very gradually drip in the oil down the side of the bowl into the yolk mixture. The mixture should emulsify and thicken.* You can drizzle a bit more quickly once the mixture is very thick.

Step 4

When all the oil has been incorporated, mix in the sugar and salt. You can add a little dashi or water to thin the mayonnaise so it will easily flow from a squeeze bottle but still hold its shape. It will thicken slightly once refrigerated. Transfer the mayo to a squeeze bottle. Refrigerate and use within 1 week.

Step 5

*If at any point the mixture breaks and separates instead of getting thick and creamy, don’t despair. Put a fresh egg yolk in a bowl and slowly whisk the broken mixture into it, as if it were the oil.