To make the pastry, whizz the butter and caster sugar in a food processor until combined. Add the egg and whizz for 30 seconds. Tip in the flour and process for a few seconds until the dough just comes together. Knead on a floured surface then shape into a flat disc and chill for 30 minutes.
Heat the oven to 160C/fan 140C/gas 3. Roll out 300g of the pastry and use to line a 21cm tart tin (freeze the rest).
Line the pastry case with greaseproof paper, fill with baking beans and bake for 15 minutes, then remove the baking beans. Return to the oven for 10 minutes until the base is a pale golden brown.
Whisk the milk and sugar together for 15 minutes with electric beaters until light and fluffy. Don’t skimp on this step as it’s important to the finished texture. Pour the milk mixture into the pre-baked pastry case then cook for 10-15 minutes until risen and the surface is tacky. Remove from oven and cool completely before slicing.