In your small pan over medium heat, combine all ingredients EXCEPT egg yolk, water and cornstarch.
Heat over medium heat until boiling whisking often. Once full boil is reached, reduce heat and simmer for 5 mins whisking occasionally.
Remove pan from heat and allow mixture to col for 10 minutes.
While sauce is cooling, whisk egg yolk and 2 tsp water together med bowl until pale yellow. About 1 to 2 min.
Whisk in cornstarch until dissolved.
Slowly drizzle hot sauce mixture from pot into egg yolk mixture in bowl, while constantly whisking. (see notes)
Cover and refrigerate until ready to use.