Preheat oven to 325°F. Lightly spray a 13 x 9 inch glass baking dish with cooking spray and line with parchment paper. Be sure to leave enough of an overhang for easy removal of the lemon bars. Set aside.
In a large mixing bowl, combine all purpose flour, powdered sugar, cornstarch and salt. Add melted butter and vanilla extract and stir just until combined.
Press the crust mixture evenly on the bottom of the prepared baking dish. Bake for 18 to 22 minutes or until the edges are starting to turn a very light golden brown.
While the crust is baking, prepare the lemon curd filling. In a large bowl, whisk eggs and granulated sugar together. Add the flour and whisk until no lumps remain.
Whisk in the half and half, lemon juice, and lemon zest.
Carefully pour the filling over the hot crust - through a fine mesh sieve if you have one - and return to the oven to bake for an additional 20 to 24 minutes or until the center is set. (Test by tapping the edge of the baking dish to see if the center jiggles at all.)
Let the bars cool completely at room temperature and then transfer to the refrigerator for another 2 to 3 hours minimum. I prefer to let them chill overnight.
When ready to enjoy, lift the bars out of the baking dish using the parchment overhang. Cut into bars and dust with powdered sugar. Enjoy chilled!
Store leftovers bars in an airtight container in the refrigerator.