In a saucepan, add the whipping cream, the coffee, and powdered sugar. Stir until all the ingredients are well incorporated on medium heat.
Add the butter and stir until melted.
Add the egg yolks and remove them from the stove while stirring fast, letting the yolk cook with the rest of the mixture.
In a bowl, whip the egg whites and the cream of tartar.
Incorporate the mix of the coffee with the egg whites slowly until smooth.
Serve in large cups and refrigerate for 3 hours.