Peel the garlic cloves, then use a microplane to grate them really fine. Alternately, put them on a cutting board and sprinkle the kosher salt on top. Then use the flat side of a chef's knife to crush the garlic, and finely chop it until it's a paste. If there are any big chunks left, they'll end up in your yuzu aioli, so make sure the garlic is very paste-like.
Put the salt and garlic paste into a bowl, then add your egg yolk and 2 teaspoons of yuzu juice (NOT all 4 teaspoons) and whisk until fully combined; I recommend using a strong milk frother.
Whisking vigorously, slowly pour in the oil a few drops at a time, increasing the pour as the oil starts to emulsify (form a mayo-like consistency).
Continue pouring and whisking vigorously until all the oil has been fully incorporated.
Then stir in the last teaspoon or two of yuzu juice and the yuzu kosho (if using), tasting as you go to determine how strong a flavor you want in your yuzu aioli.