In your small pot over medium heat, add all ingredients EXCEPT egg yolk, water and cornstarch.
Heat over medium heat until boiling, whisking often.
Once oil is reached, reduce heat and simmer for 5 mins whisking ocasionally.
Remove pan from heat, and let cool for 10 minutes.
While sauce is cooling, whisk egg yolk and water together in bowl until pale yellow, about 1 to 2 min.
Whisk in cornstarch until dissolved.
Slowly drizzle cooled hot sauce mixture from pot into the egg yolk mixture in bowl while whisking constantly. (see notes)
Cover and refrigerate until ready to use.