Soften the entire package in a large (very large) mixing bowl. If you can't get European butter or if you don't mind straying a tad from the traditional shortbread path, add a teaspoon of vanilla extract.
My guy doesn't like stuff that's really sweet, so I only add a third of a cup of powdered (caster) sugar. If you prefer your cookies with more sucrose oomph to them, make that half a cup or double it to two-thirds. Stir these two ingredients (or three, if you used vanilla extract) together until they're well blended and smooth.
I haven't tried making this recipe with gluten-free flour yet; has anyone had any success with that?
Stir the ingredients until the dough starts clumping, then use a dough cutter, also called a pastry blender, and cut the mess together until it forms a good, solid dough. It's an excellent workout for the upper arms and wrists.
For the final step, turn the shortbread dough out onto a floured cutting board and knead it a few times with your bare hands. (Clean ones, too, of course.) The heat from your hands will soften the butter further, and the pressure from your fingers will form it into a good, firm ball.
I get a good laugh out of recipes that say to grease the pan. You just dumped the better part of a half-pound of butter into that dough. Trust me, a glass pan won't need greasing.
Bake it at 325° F (160° C) for 20 minutes if you cut out the dough into little round cookies, or 45 minutes if you used the pie plate. It should be golden brown on top. The longer you bake it, the crisper it becomes.
My guy likes his with lactose-free milk. I prefer cheap plonk, well chilled. Whatever your fluid of choice, enjoy!