Rub the butter into the flour and icing sugar until the mixture resembles fine breadcrumbs.
Mix the egg yolk with lemon juice and water, then work into the flour to form a dough.
Wrap the dough in cling film and chill in the fridge for 15 minutes.
Preheat the oven to 200°C.
Roll out the pastry to 3mm thickness and line the tart tin, leaving excess over the edge.
Bake blind with parchment and beans for 15 minutes, then without for 8 minutes.
Trim the edges, patch any cracks with raw pastry, and reduce oven to 190°C.
Whisk both milks and the sugar for 10 minutes until the volume increases and becomes bubbly.
Pour the filling into the case and bake for 30 minutes until just set.