- In a saucepan off the heat, combine all the ingredients. Bring to a boil over medium heat and let simmer, stirring frequently, until the mixture thickens. Transfer to a bowl and cover with plastic wrap. Refrigerate for 1 hour or until cooled.
- With the rack in the middle position, preheat the oven to 425°F (220°C).
- In a food processor, combine the flour, baking powder and salt. Add the butter and pulse until pea-sized pieces form. In a small bowl, combine the milk and vinegar. Add to the flour mixture and pulse until the dough begins to form. Remove from the food processor.
- On a lightly floured surface, roll out two-thirds of the dough into an 11-inch (28 cm) square. Roll out the remaining dough into an 8-inch (20 cm) square. Refrigerate both pieces for 15 minutes on a baking sheet lined with parchment paper.
- With the larger piece of dough, line a square 8-inch (20 cm) metal cake pan, pressing the dough to reach three-quarters up the sides of the pan. Remove any excess dough. Spoon the filling into the crust and brush the sides with milk. With a knife, make long incisions in the centre of the second piece of dough. Gently place over the filling and press the edges to seal. Brush with milk and dust with sugar.
- Bake for 30 minutes or until golden brown and the bottom of the tart is baked through (see note). Let cool on a wire rack. Do not unmould.
- In a bowl, combine the icing sugar and water. Spoon the glaze into a pastry bag fitted with a small plain tip. Pipe the glaze onto the tart.
When the tart is done, its crust will completely come away from the pan. Lightly jiggle the pan to check the tart for doneness.