Preheat oven to 350°F (175°C).
Line an 8×8-inch baking pan with parchment paper, leaving an overhang for easy removal.
2. Make the Crust
In a medium bowl, whisk together flour, cornstarch, and salt.
Cream the softened butter and powdered sugar until light and fluffy.
Gradually mix in the flour mixture until fully combined.
Press the dough evenly into the prepared pan.
3. Bake the Crust
Bake for 15–18 minutes, or until lightly golden.
Remove from oven and let cool slightly while preparing the filling.
4. Prepare the Lemon Filling
In a medium bowl, whisk together eggs and granulated sugar until smooth.
Add flour, baking powder, lemon zest, lemon juice, milk, and vanilla extract. Mix until well combined.
5. Bake the Lemon Squares
