Stir chocolate and milk in the top of double boiler over hot water until well mixed.
Beat the egg yolk and sugar together.
Mix the cornstarch with a little of the hot chocolate mixture. Mix well and stir into the yolk and sugar. Add this to the mixture in top of double boiler and let come to the boiling point.
Remove from heat and add the cognac. Beat and let rest a few moments. Serve warm or cold over any dessert for which a rich chocolate flavor is desired.