Whisk the flour, powdered sugar and 1/4 tsp salt in a large bowl. Rub in the cold butter with your fingertips until the mix looks like damp sand with a few pea-size pieces. Stir in the egg yolk and vanilla, then squeeze the dough together. If it stays crumbly, add 1 tsp cold water.

Press the dough evenly over the base and up the sides of a 23 cm / 9-inch tart tin with a removable bottom, about 4 mm thick. Prick the base with a fork. Freeze for 15 minutes while the oven heats to 175 C / 350 F.

Set the frozen shell on a 33 x 46 cm / 13 x 18 inch sheet pan and bake for 22 minutes, until the edges are deep golden and the base is pale gold. Leave it in the tin on a rack. Start the curd in the next step while it bakes.

Combine the currants, water and 130 g of the sugar in a 3-quart / 2.8 L heavy-bottomed saucepan. Simmer over medium heat for 8 to 10 minutes, stirring often, until the berries burst and the juices turn syrupy. Blend until smooth, then press through a fine-mesh sieve into a bowl, scraping the underside of the sieve. You should have roughly 300 ml of purée. Discard the skins and seeds.

Rinse and dry the saucepan. Whisk the eggs, egg yolks, remaining 60 g sugar, cornstarch, pinch of salt and lemon juice in it until smooth. Slowly whisk in the warm purée.

Cook over medium-low heat, whisking constantly and scraping the corners, for 6 to 8 minutes. The curd is done when it's thick like pudding and a few slow bubbles break the surface. Keep whisking for 1 more minute after the first bubbles. If the edges start to set into lumps, lift the pan off the heat right away, whisk hard, and strain the curd through the sieve.

Off the heat, whisk in the cold butter a few cubes at a time until the curd turns glossy. Pour it into the baked shell, which will have cooled a few minutes in its tin, and smooth the top.

Press plastic wrap directly onto the curd and refrigerate for 3 hours, until fully set. Push up the tart tin base to unmold, slide the tart onto a plate, and slice with a hot, damp knife wiped between cuts.