In a small saucepan, combine fresh raspberries, granulated sugar, and lemon juice. Cook over medium-low heat for about 5–7 minutes until the berries break down and thicken slightly. Mix cornstarch with a splash of water, then add it to the raspberry mix, stirring until thickened. Remove from heat and let cool.

Preheat your oven to 350°F (175°C). Line an 8x8-inch baking pan with parchment paper. Blend flour, sugar, and salt in a food processor. Add chilled butter and pulse until the mixture resembles coarse crumbs. Whisk an egg yolk with a tablespoon of water and mix into the flour mixture until just combined. Press crust into the pan and bake for 15–20 minutes until lightly golden.

In a mixing bowl, whisk together granulated sugar, egg yolks, lemon zest, lemon juice, and flour until smooth. Once the crust has cooled slightly, pour the lemon mixture over it, spreading evenly.

Dollop the cooled raspberry filling over the lemon layer. Swirl the raspberry through the lemon filling for a marbled effect, avoiding disturbing the crust.

Return the pan to the oven and bake for an additional 20–25 minutes until edges are set and center has a slight jiggle. Avoid overbaking to prevent dryness.

Cool the bars at room temperature for a couple of hours. Refrigerate until fully cool, about 2–3 hours. Dust with powdered sugar, cut into squares, and serve.