Preheat the oven to 190°C/fan 170°C/gas mark 5. Put a baking sheet in the oven to get hot.
Sift the flour into a bowl. Using your fingertips, rub in the butter until the mixture resembles fine breadcrumbs. Add the egg and 1-2 tbsp water to mix to a firm dough. Roll into a ball, wrap in clingfilm and leave to rest in the fridge for 30 minutes.
On a lightly floured surface, roll out the pastry and use to line a 23cm tart tin, leaving any excess overhanging the edge. Line with baking parchment, fill with baking beans and bake blind in the oven for 15 minutes. Remove the beans and parchment and bake for a further 5-10 minutes until golden. Leave to cool then, using a sharp knife, trim the top of the pastry case.
Tip the evaporated milk and sugar into a large bowl and, using an electric hand mixer on full power, whisk together for 15 minutes until the mixture resembles a light coffee-coloured, creamy foam that doesn’t quite hold peaks.
Pour the mixture into the tart tin and bake for 20 minutes on the hot baking sheet. When cooked, the filling should be lightly set with a sticky surface. Leave to set overnight in the fridge.