In a small pot, mix white miso, mirin, and sugar. Bring to boil over high heat, stirring occasionally.
Cook for additional 1 minute while boiling, and add egg yolk. Whisk immediately after adding yolk. Remove from heat.
Add vinegar, stir, and strain the mixture with strainer.
Add karashi (Japanese mustard) and sesame oil to taste.
Enjoy with vegetables (carrots, cauliflower, green peppers, broccoli, cucumber, Japanese radish, etc).
*1 Mirin can be substituted with Sake and Sugar, 4:1 ratio