Place all ingredients into a food processor and pulse to combine. The mix will look like it won't form a dough, but stick with it and you'll get a result like the above photos.
I imagine this will work with a stand mixer or an electric whisk too, but I haven't tried those methods. My mother-in-law says if you don't have a mixer, you can just cut the butter into small cubes, then rub the mixture together with your fingertips until the butter is incorporated and a dough forms.
Press the dough into a rough 6 inch circle on a baking tray or cake pan (as pictured). You don't need to grease the pan. Score the dough into slices (don't cut all the way through, just mark the lines) and prick all over with a fork.
If you like, you can crimp the edges as I did. (Petticoat shortbread is supposed to be baked in a fluted tart tin to achieve the ruffles. I made do.)
Bake for 25 to 30 minutes, until the edges of the shortbread are only very lightly golden. You want the shortbread to remain as pale as possible. Check at 25 minutes to see where you are.
Remove from the oven and re-score the dividing lines if necessary. Allow the shortbread to cool in the pan for 15 minutes or until it firms up a bit.
Slice the petticoat pieces through the pre-scored lines and remove the cookies to a wire rack to cool. You can dust the cookies with a bit of sugar for sparkle if you want.
Cookies will keep in a sealed container for about a week, but ours only lasted three days.
Recipe by The Bright Kitchen at https://www.thebrightkitchen.com/petticoat-shortbread/