Steep instant espresso in boiling water and cool to room temperature.
Mix it with egg yolks and sugar and set in the top of a double boiler.
Beat with an electric beater, until mixture is thick and fluffy.
Beating constantly, pour the rum into the egg yolk mixture in a slow thin stream and continue beating, until mixture almost triples in volume.
:- This may take 10 mins or so.
Spoon mixture into balloon wine glasses or goblets or chill for 30 mins.
:- Serve as a sauce over ladyfingers or angel food cake.
:- This goes very nicely with vanilla ice cream and cheese cake.