Meanwhile, put the raspberries in a pan with the orange zest and juice and heat until the raspberries start to give off their juice. Add the sugar and stir it in. Taste the juice and add more sugar if you need to, then push the lot through a sieve into a clean pan. Add a few tbsp of the mix to the cornflour and mix it until there are no lumps. Stir this into the pan. Turn on the heat and whisk the mixture until it starts to bubble and thicken. Add the butter, off the heat, and whisk it in, then whisk in the egg yolks. Put the pan back on the heat and whisk until the mixture thickens. Allow to cool.