Position oven rack in the middle and preheat to 350°F (175°C). Line an 8x8-inch (20x20cm) light-colored aluminum pan with parchment paper, leaving at least 1 inch of overhang on all sides. Lightly butter the parchment.

In a large bowl, whisk together 1½ cups (190g) all-purpose flour, ½ cup (100g) granulated sugar, and ¼ tsp fine sea salt. Add ¾ cup (170g) cold cubed unsalted butter and work it into the flour using a pastry blender or fingertips until the mixture resembles coarse, damp sand with pea-sized butter pieces still visible — approximately 2 to 3 minutes. Do not overwork.

Set aside approximately one-third of the mixture (about 1 heaping cup / 200g) in a small bowl for the streusel topping. Keep both portions refrigerated while preparing the fillings.

Press the larger crust portion firmly and evenly into the bottom of the prepared pan using the flat bottom of a measuring cup, working it into all four corners to an even ¼-inch (6mm) thickness.

Bake the crust at 350°F (175°C) for 15 to 18 minutes until the edges are just beginning to turn pale golden and the surface looks dry and matte. Remove from the oven and cool on a wire rack for exactly 10 minutes before adding any filling.

While the crust bakes and cools, make the lemon curd: whisk together 2 large eggs, 1 egg yolk, ½ cup (100g) granulated sugar, ⅓ cup (80ml) fresh lemon juice, and 1 tsp lemon zest in a small heavy-bottomed saucepan. Cook over medium-low heat, whisking constantly, until the mixture thickens enough to coat the back of a spoon and hold a line — approximately 8 to 10 minutes. Remove from heat and whisk in 2 tablespoons (28g) cold cubed butter until fully melted and smooth. Strain through a fine mesh sieve if desired. Set aside to cool for 5 minutes.

Make the raspberry layer: combine 1½ cups (180g) fresh raspberries, 3 tablespoons (36g) granulated sugar, and 1½ tablespoons (12g) cornstarch in a small saucepan over medium heat. Cook, stirring constantly and gently pressing the berries, until the mixture is thick, jammy, and holds its shape briefly when dragged with a spatula — approximately 5 to 7 minutes. Remove from heat and set aside for 5 minutes.

Pour the lemon curd over the cooled crust and spread evenly to the edges. Gently spoon the raspberry mixture over the lemon curd in small spoonfuls and spread carefully from the center outward — do not swirl or mix the two layers. Remove the reserved streusel from the refrigerator and scatter it in loose, irregular crumbles over the raspberry layer without pressing down.

Refrigerate the assembled pan for 10 minutes, then bake at 350°F (175°C) for 25 to 30 minutes, until the streusel is pale golden and the center no longer wobbles as a liquid when the pan is gently shaken. Do not overbake.

Remove from the oven and cool on a wire rack for 1 hour at room temperature, then transfer to the refrigerator and chill for a minimum of 2 hours — overnight is strongly preferred. Do not slice before the full chill time.

Lift the chilled bars from the pan using the parchment overhang. Using a sharp chef's knife rinsed under hot water and wiped clean between each cut, slice into a 3x3 grid for 9 bars. Dust with powdered sugar immediately before serving if desired.