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. Use On Heirloom Chocolate Cake.