Mash the banana, then put it in a saucepan with the sugar and cook on a medium-high heat, stirring, for five minutes, until the mix starts to bubble and brown a little.
Turn down the heat to very low, then whisk in the egg yolks and carry on cooking on a low heat, stirring constantly and scraping the sides and bottom of the pan with a spatula, until the curd bubbles and thickens enough to coat the back of a spoon, or reaches 75C.
Take off the heat, then stir in the butter and, if using, the vanilla extract and rum.
Pour the curd into a sterilised jar, leave to cool (assuming you can resist the temptation of the warm curd), then seal and refrigerate.
