Heat the milk in a saucepan over medium heat and bring almost to the boil. Remove from the heat. Beat the egg yolks and sugar at medium speed in the bowl of an electric mixer until pale and thick, about 5 minutes. With the mixer on low speed, sprinkle the Maizena over the egg yolk mixture. Beat on medium speed until smooth. With the mixer on low speed, slowly pour the hot milk into the egg mixture. Pour the milk/egg mixture into the saucepan and cook over a low heat, stirring constantly, until the mixture thickens, about 5 to 7 minutes. (Be careful, as the custard will thicken and then quickly become scrambled eggs, so stay close to the pan during this time!) Stir in the vanilla, butter and whipping cream, as well as the ingredient of your choice if you want to give your custard a different flavour (I'm not giving you the quantities for this, it's up to you to test according to your personal taste). Strain the custard and refrigerate until cold.