Scrape back into the bowl and press a sheet of Clingfilm onto the surface and refrigerate until needed. Pour the mixture back into the pan and over medium heat cook, whisking constantly, until the mixture is very thick. When the milk is at temperature pour over the egg yolk mixture and immediately whisk together, to prevent the eggs from curdling. Meanwhile place the egg, yolks, sugar, cornflour and pistachio paste into a large bowl and whisk together to combine. To make the pistachio filling place the milk into a medium sized saucepan and bring to the boil.