Take the butter out ahead of time and cut it into cubes so it softens. Set a bowl of hot but not boiling water on the stove, and place a bowl with the egg whites and sugar on top. Stir constantly until the sugar fully dissolves - there should be no grains left.

Remove the bowl from the water bath, pour the whites into a dry, clean, grease-free mixing bowl, and beat with a mixer until the volume roughly triples and the mixture cools completely.

Continuing to beat, add the soft butter gradually, a tablespoon at a time. The cream will be runny at first but will become dense toward the end.

Beat the cream to the desired consistency, flavor and tint it if you like, and use as needed.