Mix sugar, yolks, and corn syrup in small bowl.
Simmer cream in heavy 3 qt saucepan.
Add both chocolates. Stir over low heat until smooth.
Remove from heat. Stir in butter, then sugar mixture.
Cook over low heat until temperature is 165-170~(about 10 minutes).
Stir constantly. DO NOT SCRAPE SIDES OF BOWL
(or candy monster will bite) Add vanilla.
Beat with mixer until cool. Let stand until spreadable, about 1 1/2hrs.