Prepare the Tart Dough: In a large bowl, combine the 250g (2 cups) all-purpose flour and 50g (1/4 cup) granulated sugar. Add the 125g (1/2 cup + 1 tbsp) cold, cubed unsalted butter. Using your fingertips or a pastry blender, cut the butter into the flour mixture until it resembles coarse breadcrumbs with some pea-sized pieces of butter remaining. This is crucial for a tender crust.

Add the egg yolk and 30ml (2 tbsp) of ice water to the bowl. Mix with a fork until the dough just starts to come together. If it seems too dry, add more ice water, 15ml (1 tbsp) at a time, until the dough forms a cohesive ball. Be careful not to add too much water.

Gently shape the dough into a flat disc, wrap it tightly in plastic wrap, and refrigerate for at least 30 minutes (or up to 2 days). Chilling the dough allows the gluten to relax and the butter to firm up, making it easier to roll and preventing shrinkage during baking.

Preheat your oven to 190°C (375°F). Lightly grease a 23cm (9-inch) tart pan with a removable bottom. On a lightly floured surface, roll out the chilled dough into a circle approximately 3mm (1/8 inch) thick and large enough to fit your tart pan, with a slight overhang. Carefully transfer the dough to the tart pan, pressing it gently into the bottom and up the sides. Trim any excess dough from the edges.

Prick the bottom of the tart shell all over with a fork. Line the tart shell with parchment paper and fill it with pie weights or dried beans. This is called blind baking and prevents the crust from puffing up unevenly.

Bake the tart shell for 15 minutes at 190°C (375°F). Carefully remove the parchment paper and weights. Return the tart shell to the oven and bake for another 10-12 minutes, or until the crust is golden brown and appears dry. If any part of the crust is browning too quickly, you can loosely tent it with aluminum foil. Remove from oven and let cool slightly while you prepare the filling.

Prepare the Lemon Filling: In a medium saucepan (off the heat), whisk together the juice and zest of 4 large lemons (about 200ml juice, 4 tbsp zest), 200g (1 cup) granulated sugar, and 4 large eggs. Whisk until thoroughly combined and smooth.

Place the saucepan over medium-low heat. Cook, stirring constantly with a whisk or a heatproof spatula, until the mixture thickens enough to coat the back of a spoon. Do not let it boil, as this can scramble the eggs. This usually takes about 8-10 minutes. The mixture should reach approximately 77°C (170°F) on an instant-read thermometer if you have one.

Remove the saucepan from the heat. Gradually whisk in the 100g (7 tbsp) softened unsalted butter, one piece at a time, until fully incorporated and the filling is smooth and glossy. The butter adds richness and a silky texture to the lemon curd.

Pour the warm lemon filling into the pre-baked tart shell. Smooth the top with a spatula. The filling will continue to set as it cools.

Let the tart cool completely at room temperature for about 1-2 hours. Once cooled, carefully transfer the tart to the refrigerator and chill for at least 2-3 hours, or preferably overnight, to allow the filling to set completely.

To serve, carefully remove the outer ring of the tart pan. Dust with powdered sugar if desired. Slice with a sharp knife and enjoy the bright, tangy flavor of your homemade lemon tart.