zest of 1 lemonpeeled with a vegetable peeler (just the yellow part, not the bitter white)
Set the oven to 350F
First make the lemon sugar. Put the sugar into the bowl of a small food processor, along with the zest. Process, scraping down the sides if necessary, for about a minute or so, until the zest is completely incorporated into the sugar. Measure out 1/2 cup for this recipe, and pack it down, like you do with brown sugar, when you measure it.
Put the sugar into a mixing bowl and add the flour, oat flour, and cornstarch. Whisk to combine well.
Pour the melted butter into the dry ingredients and mix until well combined.
Pat the dough into an 8 inch square baking pan, smoothing it flat with your fingers. I line the pan with parchment paper and leave the ends long so I can pull it out for cleaner cutting later. Press the dough gently, but flatten it as best you can.
Prick all over with the tines of a fork. Some people like to sprinkle the top with a little sugar, I didn't.
Bake for about 22 minutes, or until it is just beginning to turn pale golden around the edges. It will still be quite pale overall. If you bake it longer, it will be crisper, but I like the soft melting texture I get after about 22 minutes.
Let the shortbread cool for a few minutes, and then use a sharp knife to cut it into squares, or 'fingers'. Cutting the shortbread while still warm makes a cleaner cut.