Preheat your oven to 375°F (190°C). Line a 9x13 inch baking pan with parchment paper, leaving an overhang on the sides for easy lifting.

In a large mixing bowl, whisk together the flour, 1/2 cup granulated sugar, baking powder, and salt.

Add the cold cubed butter to the dry ingredients. Use a pastry blender, two forks, or your fingertips to cut the butter into the flour mixture until it resembles coarse crumbs with some pea-sized pieces remaining.

Remove 1 cup of this mixture and set aside for the crumble topping. To the remaining mixture in the bowl, add the 1/4 cup milk and mix gently until just combined and a soft dough forms.

Press the dough evenly into the bottom of the prepared 9x13 inch baking pan. Bake for 12-15 minutes, or until lightly golden. Remove from oven.

While the crust is par-baking, in a medium saucepan, combine the sliced strawberries, 1/2 cup granulated sugar, and lemon juice. Cook over medium heat, stirring occasionally, until the strawberries begin to soften and release their juices, about 5-7 minutes.

In a small bowl, whisk together the cornstarch with 2 tablespoons of cold water to create a slurry. Stir the cornstarch slurry into the simmering strawberry mixture. Continue to cook, stirring constantly, until the mixture thickens, about 1-2 minutes. Remove from heat.

Pour the thickened strawberry filling evenly over the warm, par-baked shortcake crust in the pan.

Sprinkle the reserved 1 cup of shortcake crumb mixture evenly over the strawberry filling.

Return the pan to the oven and bake for another 30-35 minutes, or until the crumble topping is golden brown and the strawberry filling is bubbly.

Remove from the oven and let the bars cool completely on a wire rack before cutting. This is crucial for the bars to set properly, about 1-2 hours.

Once cooled, lift the bars out of the pan using the parchment paper overhang and cut into 12 squares or rectangles.