Boil the coffee with the required amount of water but don't sweeten it. If boiling it in a Turkish coffee pot, strain it.
Beat the butter with the yolk well and begin adding the coffee in small amounts while stirring nonstop and being careful not to curdle the butter - to prevent curdling, the coffee and butter need to be the same temperature. Flavor the cream with vanilla and liqueur.
If you notice the butter having an unpleasant smell, you can grate a little nutmeg into it. Sweeten the prepared cream with the sugar.