Combine the flour, sugar, salt, and lemon zest in a large mixing bowl. Add the butter and mix with a pastry blender or your fingers until the mixture resembles coarse crumbs. Add the egg yolk and ice water and work that in with your hands. (Or do the whole thing in a food processor, pulsing a couple of times to combine the dry ingredients, then pulsing in the butter, and then the egg.) Check the consistency of the dough by squeezing a small amount together between thumb and forefingers: You want there to be just enough moisture to bind the dough so that it holds together without being too wet or sticky. If it's still crumbly, add a little more ice water, 1 teaspoon at a time. When you get it to the right consistency, shape the dough into a disk and wrap it in plastic. Put it in the refrigerator and chill for at least 30 minutes.