Heat the oven to 350°F. In a microwave-safe bowl heat the butter until just melted, 1 minute or so, depending on the strength of your microwave. (Don't melt it until it's oil, stop when there's a hunk of semi-solid butter at the center. It will melt as you make the dough.) Add almond flour, sugar, cornstarch, and salt. Using a rubber spatula, stir to combined and solid. (The shortbread dough will feel like Play-doh.)
Press the dough into a 9-inch cake pan or 8-inch square. Prick all over with the tines of a fork. Transfer to oven and bake until browned and set, about 20 minutes.
Meanwhile, IN THE SAME BOWL combine the sugar, cornstarch, salt, eggs, lemon juice, and lemon zest. Whisk to combine and make sure to stir until any and all clumps of cornstarch are incorporated. (You can use a whisk or a handheld mixer.) Pour into the still-warm crust.
Bake until set and it starts to pull away from the sides of the pan, 20 to 25 minutes. Remove to let cool for several hours uncovered. Shake over Confectioner’s Sugar to garnish.