Place 2 cups raspberries in a small saucepan over medium heat and smash gently. Stir constantly until the berries release juices and begin to simmer. Cook until the volume reduces by about half and the puree thickens slightly, then strain through a fine mesh to remove seeds. Reserve 1/4 cup reduced raspberry puree for the lemon layer.

Preheat oven to 350°F (175°C). Whisk together flour, sugar, corn starch, and salt in a bowl.

Pour in melted unsalted butter and stir until a crumbly dough forms. Press evenly into a lined 9x13-inch pan.

Bake until the edges are golden, about 18–22 minutes, and let cool slightly.

Lower oven to 325°F (160°C). In a large bowl, whisk sugar with corn starch. Whisk in eggs until smooth.

Add lemon juice and reserved raspberry puree; whisk until fully combined.

Pour the lemon-raspberry mixture over the warm shortbread base and return to oven.

Bake until the filling is set at the edges but wobbles slightly in the center, about 25–30 minutes. Cool to room temperature, then chill for at least 3 hours before slicing.