Using your hands, rub the citrus zest into the sugar to release the oils. It should smell wonderful.
Cream the sugar and salt together with the butter, either by hand or with a mixer.
Add flour in 1/4 cup increments and mix until thoroughly each time. It should be crumbly and fairly uniform when you are done. No big chunks of butter should remain. You may need to scrape the sides of your bowl a few times.
Lay out a large sheet of plastic wrap and gently pour the mixture onto it. Using your hands and the wrap, form the dough into a log about 10 inches long. You need to do this very firmly, you want to really compress it. It helps to push with one hand and pull up on the wrap from underneath simultaneously.
When you have a nice, firm log, wrap again in another layer of plastic wrap; really twist the ends to make it as firmly compressed as you can. If you have one, here's a trick I learned from Dorie Greenspan: stick your wrapped, compressed log into a paper towel roll. It's just the right size, makes the cookies super even, and helps hold the dough together in the fridge. You can also push both ends together once you get it into the tube, which helps compress it even more.
Refrigerate your log for at least two hours. I've waited up to 36 hours before slicing it and there's no difference in delicious.
When you are ready to bake, heat your oven to 350°F and line a baking tray with parchment paper. I've tried with silipat, but the cookies melted, I'd stick with parchment.
Remove your log from the fridge, unwrap, and slice it into about 20 cookies (slices are a chunky 1/4"). Place the cookies on the baking tray; these cookies don't spread at all in the oven, so you can get them all on one tray.
Bake for 12-15 minutes, rotating your tray halfway through. The cookies should have a lovely golden brown around the bottom edges. Remove from oven and let cool for a few minutes on the tray before transferring to a cooling rack.