Mix matcha powder and milk well.
Pour the mixed ingredients into a small saucepan and heat over medium, stirring.
Combine egg yolk (onnao) and sugar in a bowl and mix.
Slowly add the warmed matcha milk to the egg yolk mixture, stirring constantly.
Once everything is well mixed, return it to the saucepan and cook over low heat, stirring, until it thickens.
Pour into a cup, let cool, and serve.