Get a measuring cup to make the mayo and keep a hand blender ready.
Add whole egg, sugar, salt, vinegar and oil to the measuring cup.
Immerse the hand blender into the cup and turn it on but don't make any sudden moves. Just keep it in one place, do not move up blender stick or else the mayo won't turn out great. Literally keep it in one place!
Keep it immersed and you will notice that the mayonnaise will turn whiter and silkier over time. Blend until you are satisfied with the consistency and color. This can take about 3 minutes.
You can choose to flavor your japanese mayonnaise at the end with mustard or sriracha. Mix in well.
Store your mayonnaise in an airtight container until further use or use right away. See serving recommendations in post.
Video
Notes
Use only warm room temperature egg, never a cold egg. A cold egg turns the mayonnaise more liquid. This is important!
We used brown sugar but you can use regular sugar too.
Substitute rice vinegar with distilled vinegar if you can't get rice vinegar.
Recipe quantity comes to 7 ounces/ 200 grams. That's about 5 servings if used in sushi and as a dip.
You can flavor your mayo. See in post for flavor ideas.