Prepare Raspberry Puree: Combine 2 cups of raspberries in a saucepan over medium heat. Cook until they break down and release their juices, stirring occasionally, about 5-7 minutes. Strain through a fine sieve to remove seeds, reducing it down to about 1/4 cup of puree.
Make the Crust: Preheat your oven to 350°F (175°C) and line a baking dish with parchment paper. In a mixing bowl, combine flour, sugar, corn starch, and salt before pouring in melted butter. Stir until fully incorporated and press this mixture into an even layer across the bottom of your prepared baking dish.
Bake the Crust: Place the crust in your preheated oven and bake for about 20 minutes until it’s lightly golden brown around the edges.
Prepare Lemon Raspberry Custard Filling: While the crust is baking, whisk together eggs, granulated sugar, corn starch, lemon juice, and reduced raspberry puree in another bowl until smooth and well combined.
Combine Layers: Once your crust is baked and slightly cooled, carefully pour the lemon raspberry custard over it evenly. Bake again for an additional 25-30 minutes until set but still slightly jiggly in the center.
Cool and Serve: Remove from oven and allow to cool completely at room temperature before refrigerating for at least two hours to set properly. Once chilled, cut into squares and serve.