Melt chocolate pieces, coffee, and sugar together in top part of double boiler over hot water.
Stir until completely smooth.
Remove from heat.
Beat in egg yolks, one at a time.
Return to heat and cook until thickened.
Cool to lukewarm.
Beat butter into chocolate mixture until thoroughly blended and satiny.
Cool or chill to spreading consistency.
Marked lines and tap gently with another knife to crack caramel.