In a large mixing bowl, combine cold, cubed butter with plain flour and a pinch of salt. Rub the mixture until it resembles coarse breadcrumbs. Gradually add cold water, one tablespoon at a time, mixing until a soft dough forms. Shape into a disc, wrap in cling film, and refrigerate for 30 minutes.

Preheat the oven to 180°C (350°F). Roll out the chilled pastry and fit it into a tart tin. Prick the bottom, line with parchment paper, fill with baking beans or rice, and bake for 15-20 minutes until lightly golden. Remove beans and paper, then bake for an extra 5 minutes.

In a medium saucepan, melt the butter over low heat. Add Demerara sugar, stir until dissolved. Gradually whisk in milk, sift in plain flour, and stir until the mixture thickens and bubbles gently, about 5-7 minutes. Remove from heat and let cool.

Pour the cooled filling into the baked pastry shell, tap the tart gently to remove air bubbles, and create a smooth top.

Cover loosely with cling film and chill for at least 2 hours until set.