To make the custard, put the whole milk in a large pan over a medium heat and bring to a simmer. Meanwhile, put the cornflour, caster sugar, vanilla and eggs into a large bowl and whisk until smooth. Pour the hot milk over the egg mixture, whisking together. Pour the custard back into the pan and cook gently, whisking constantly, until thickened. Divide the custard equally between 2 bowls. To one, add the 50g chopped dark chocolate and sea salt, whisking until the chocolate has melted. To the second, add the mascarpone and Baileys, whisking until evenly combined. Press a sheet of clingfilm onto the surface of the custards to stop skins forming and chill for a few hours.