1. In a saucepan, heat the coffee and incorporate the butter while whisking. Add half of the sugar and whisk for 1 minute.
The mixture should not boil. Remove from heat and set aside.
2. Separate the egg whites from the yolks. Whisk the egg yolks and the remaining sugar until the mixture turns pale.
3. Pour the coffee preparation into the yolks and mix.
4. Beat the egg whites until stiff peaks form. Pour into the coffee preparation and mix gently.
5. Pour the preparation into a cup or into ramekins and refrigerate for 1 hour before serving.