In a small saucepan, combine the 2 cups of raspberries with a splash of water. Cook over medium heat, stirring occasionally until the berries burst and the mixture thickens, about 10-15 minutes.

Strain through a fine sieve to remove the seeds and allow cooling.

Preheat your oven to 350°F (175°C).

In a large mixing bowl, combine 2 1/4 cups flour, 1/2 cup sugar, 1 tablespoon corn starch, and 1/4 teaspoon salt.

Stir in 1 cup melted unsalted butter until the mixture resembles wet sand.

Press firmly into the bottom of a greased 9x13 inch baking dish.

Bake for 15-20 minutes, or until the edges turn golden brown. Remove and let cool slightly.

In another bowl, whisk together the 1 1/2 cups sugar and 1/3 cup corn starch.

Add the eggs one at a time, mixing well after each addition.

Stir in the reduced raspberry puree and 3/4 cup lemon juice. Mix until fully combined.

Pour the lemon raspberry mixture over the cooled shortbread base.

Bake for an additional 20-25 minutes, or until set and slightly golden on top with firm edges and a slight jiggle in the center.

Once baked, remove from the oven and allow to cool completely.

Cut into squares, dust with powdered sugar if desired, and serve.