To make the custard, mix 3 tbsp of the milk with the cornflour in a small bowl until smooth. Heat the remaining milk in a heavy-based pan until steaming. Put the remaining custard ingredients, along with the cornflour mixture, into a heatproof bowl, and whisk to make a golden paste. Pour the hot milk over the yolks, continually stirring, until smooth. Transfer back to the pan and heat slowly, stirring for 5-8 minutes or until thickened – it should thickly coat the back of a wooden spoon. Remove from the heat, strain into a clean bowl and cover the surface with baking paper. Leave to cool while you blind bake the pastry – it will thicken more as it cools.