1. Make and Reduce Raspberry Puree:

• Add the 2 cups of fresh or frozen raspberries to a blender or food processor. Pulse until completely smooth.
• Place a metal sifter or fine-mesh strainer over a bowl. Pour the puree into the sifter, a little at a time, and use a spatula or spoon to push the puree through, leaving the seeds behind. Discard the seeds. (This step is optional but recommended for a smooth filling).
• Add the seedless puree to a small saucepan. Place over low-medium heat. Bring to a gentle simmer and cook, stirring occasionally, allowing the excess liquid to evaporate until the puree is much thicker and reduced to about 1/4 to 1/3 cup total volume. This will take approximately 15-20 minutes.
• Remove the reduced puree from the heat and set it aside to cool completely while you make the base.

2. Make and Bake the Shortbread Base:

• Preheat the oven to 325°F (160°C).
• Line a 9×13 inch baking pan with parchment paper, leaving an overhang on the longer sides for easy lifting later.
• In a medium bowl, whisk together the all-purpose flour, 1/2 cup granulated sugar, 1 tablespoon corn starch, and salt.
• Pour the melted butter over the dry ingredients. Stir with a fork or spatula until combined. The mixture should be thick and somewhat crumbly but hold together when pressed.
• Press the dough evenly into the bottom of the prepared parchment-lined pan. Use your fingers or the bottom of a measuring cup to create a firm, even layer. Create a slight lip (about 1/4-1/2 inch high) around the edge of the pan by pressing the dough up the sides; this helps contain the filling.
• Bake in the preheated oven for 20-25 minutes, or until the top looks set and the edges are just beginning to turn lightly golden.
• While the base bakes, make the filling.
• Once baked, remove the pan from the oven. Immediately (while hot), gently prick the top surface of the shortbread all over with a fork. Do not prick all the way through to the bottom, just the very top layer – this helps prevent the base from puffing up too much when the filling is added.

3. Make the Lemon Raspberry Filling:

• In a large bowl, whisk together the 1 1/2 cups granulated sugar and 1/3 cup corn starch until no lumps remain.
• Whisk in the eggs one at a time until well combined.
• Whisk in the cooled, 1/4 cup of reduced raspberry puree.
• Carefully whisk in the freshly squeezed lemon juice until the mixture is smooth. It will be quite liquidy.

4. Assemble and Bake the Bars:

• Pour the liquid lemon raspberry filling evenly over the warm, pricked shortbread base in the pan.
• Carefully return the pan to the oven (still at 325°F / 160°C).
• Bake for another 20-25 minutes, or until the filling looks set and the center doesn’t jiggle excessively when the pan is gently nudged.

5. Cool and Chill Completely:

• Remove the pan from the oven. Place it on a wire rack and let the bars cool completely in the pan at room temperature for at least 1 hour.
• Once at room temperature, cover the pan loosely with plastic wrap or foil.
• Place the covered pan in the refrigerator to chill for at least 2 hours (or preferably longer, even overnight) until the filling is completely firm and cold. Chilling is essential for clean slicing.

6. Slice and Serve:

• When ready to slice, use the parchment paper overhangs to carefully lift the entire slab of bars out of the pan onto a cutting board.
• Use a large, sharp knife to cut the slab into squares or rectangles. For the cleanest cuts, wipe the knife blade clean with a damp paper towel after each cut.
• Store leftover bars covered in the refrigerator.