In a small saucepan, whisk the whole milk and granulated sugar and bring just to boiling point. Remove from the heat and allow to steep for five minutes.

While the milk is steeping, in a small bowl combine the egg yolks with the cornstarch.

Temper the egg mixture by slowly whisking in a ladle or two of the hot milk mixture into the egg mixture. Do so slowly so the eggs don't scramble.

Add the tempered egg mixture back into the saucepan with the hot milk. Heat up again over medium heat and whisk constantly until the mixture slightly thickens.

Optional: If adding rum, whisk it in once the sauce is done.

Remove from the heat and allow to cool slightly before serving.