Cut chocolate in small pieces.
Combine with sugar, milk and corn syrup, and cook slowly, stirring often, to 238° F. or soft ball stage.
Then remove from heat, cool until lukewarm or 110° F.
Add butter and vanilla and beat until creamy and thick enough to just hold its shape when dropped from spoon.
Spread on cake quickly.
Makes enough to fill and frost 1 cake or 10 cupcakes.