Mix flour, sugar, cornstarch, and salt in a food processor until blended, about 20 seconds.
Add lemon zest and lemon juice.
Cut butter into tablespoon-size pieces. Add to processor and pulse until dough resembles a coarse meal. The dough will form a ball if pressed together but seem crumbly in the bowl.
Press dough into an 11-inch tart pan with a removable bottom. A 10-inch cake pan can also be used if desired. Ensuring the dough is pressed evenly will create a firm shortbread.
With a sharp knife, score the shortbread into eight wedges, making sure to not cut all the way through.
Place on a parchment-lined sheet pan and bake in a preheated 300°F oven for 40-50 minutes (50-60 minutes if using a cake pan). When done baking, the shortbread will be firm to the touch around the edges.
Remove from oven and cut shortbread while still warm, using the scored lines as a guide.
Once cool, remove the shortbread from pan and enjoy!