Blend egg yolks and mustard for 20 seconds with a food processor or immersion blender.
With the processor running, drizzle in about ½ cup oil in a thin stream. Go slow so it emulsifies properly.
Add salt, sugar, and dashi powder. Blend briefly to mix.
Keep blending and drizzle in another ½ cup oil.
Add vinegar and lemon juice. Drizzle in the last ½ cup oil and blend for 10 seconds, just enough to combine. Don't overblend or it might separate.
Add more sugar, salt, or lemon juice if needed.
Store in an airtight container in the fridge.
