For the crème mousseline, warm the milk in a large pan over a medium-high heat until steaming. Meanwhile, put the yolks, vanilla, sugar and cornflour into a bowl and whisk together for a couple minutes or until it forms a smooth, pale paste. Gradually pour in the milk, whisking as you do to prevent curdling. Scrape the custard back into the pan and cook over a medium heat, whisking constantly, until the custard is thickened and bubbling. Cook for 2 minutes more to ensure the cornflour is cooked out, then scrape into a bowl. Add 50g of the butter and quickly stir together to combine. Cover with a piece of baking paper touching the surface and refrigerate for a few hours until thoroughly chilled.