Spray a 9x13 baking dish with nonstick spray, then line with parchment paper, making sure there's an overhang on both of the longer sides (this really helps for easy removal.)

Place the flour, corn starch, salt, and powdered sugar in the bowl of a food processor; pulse a few times to incorporate. Add the butter and blend until the mixture resembles crumbs, 10 to 15 seconds. Sprinkle the mixture into the prepared pan and press firmly with your fingers into an even layer and a little up the sides (this keeps the filling from spilling beneath the crust).

Refrigerate for 30 minutes (or freeze for 15 minutes.)

Preheat oven to 350 degrees F with rack in the center position.

Bake the crust until just beginning to brown slightly at the top edges, 15 to 20 minutes.

Return pan to the oven and bake another 20-25 minutes, or until the top is slightly browned and the custard looks set and firm. Let the bars cool completely to room temperature; it will take several hours.

Once room temp, pop them in the fridge until cold and firm.

To cut, use the parchment overhang to lift the rectangle out of pan and onto a cutting board. Carefully loosen the parchment from the edges of the crust.

Using a sharp knife, cut into even squares. (For really clean cuts, slightly dampen the blade before cutting and clean it off in between each slice, using one downward motion to cut through.)

Dust the squares with powdered sugar, using a fine sieve.