In a small sauce pan on low/simmering heat, combine vinegar and dashi/hondashi.
Stir well while simmering to evaporate some water concentration and reduce the amount of liquid down to about less than/equal 1 Tbsp.
Remove from heat, add sugar and mustard. Stir to mix and let cool to room temperature.
In a clean container, add egg yolk and 1/2 tsp salt, and whisk them until yolk color goes light. (Perfect job for a hand blender/mixer. Note that you could also throw them in a blender/food processor and pulse for a few times to achieve same result)
While whisking/machine blending/mixing, slowly dripping avocado oil, spoon-by-spoon every 5-10 second to let mixture become thick and emulsifying.
When mixture reach full fluffy thickness, start adding the vinegar/dashi/sugar mixture to let it soften up. Whisk/pulse several times to combine. This is your home made Kewpie (Japanese) Mayo.