In a heavy, medium saucepan, bring butter, milk, and light cream to a boil.
Meanwhile, in a separate bowl, combine sugar, cornstarch, and eggs. Whip together until ribbons form (the mixture should feel thicker, but lightened in color. Lift up the whisk and the mixture that falls off onto the top of the mixture resembles a ribbon, before it disappears). At this stage, the mixture should be fuller, with significant expansion, and almost foam-like.
When the cream mixture starts to boil, whisk in the egg mixture, and return to a boil, boiling for one minute.
Pour into a chilled bowl, cover the surface with plastic wrap to prevent a skin from forming or, like me, just stir the skin back into the custard before using.
Chill overnight when possible. Otherwise, chill well and then, using a whisk, mix in the dark rum.