Steps adapted from The Cake Bible

Makes enough to ice two 9" cake layers

6 large egg yolks
3/4 C sugar
1/2 C corn syrup
2 C unsalted butter (soft)
6 oz melted bittersweet chocolate

Have a greased heatproof glass measure standing by.
Beat yolks until pale and thick.
Heat sugar and syrup until rolling boil (bubbles cover entire surface). Immediately pour into glass measure to stop cooking.
With mixer runner, drizzle hot syrup into eggs, avoiding the paddle (beaters).
Beat until completely cool.
Add butter in small chunks and beat until blended.
Add chocolate and beat until blended.

Bring to room temperature before using as icing.