In medium-sized heavy saucepan, melt broken chocolate with water and coffee.
Stir constantly until mixture is very smooth.
Remove from heat.
Mix in sugar and beat until smooth.
Beat in egg yolks and vanilla.
Place pan in bowl of ice and water.
Add butter, in four additions, beating until frosting is very light in color and thick enough to spread.
Frost Bonanza King Devil's Food Cake or other favorite chocolate cake.
Chill to set frosting.