Combine the fructose, milk, cornstarch, and cocoa powder in a small saucepan.
Bring to the boil stirring constantly.
Remove from the heat and stir until smooth and thick about 3 minutes.
Using an electric mixer, beat the egg yoke, vanilla, and salt and the chocolate mixture
Beat until smooth.
Add the butter and margarine, one tablespoon at a time beating after each addition until the frosting has the consistency of whipped butter.
Used to frost one cake: Place in refrigerator until ready to serve.
