Original Scottish/Irish Recipe has just 3 ingredients. (These taste better the next day ...and get even better going forward from there).
5 Cups white flour/All-purpose flour
5 sticks real butter, softened (NEVER use margarine, it'll change the texture/flavor dramatically)
2 cups white cane sugar (can be increased up to 2 1/2 cups or decreased down to 1 cup with good results either way. ).
If you decide to include the optional vanilla extract (below), then go with the 2 cups sugar to help the vanilla "blossom" in flavor. I used 2 cups sugar and thought it was great. The vanilla became more pronounced the next day.
Both of the following are optional, but I like them in this recipe).
1 tsp. EACH: Baking Soda and Baking Powder. Add to flour bowl. Leavening makes the bars a bit fluffier or softer. Without it, bars become very solid, very "packed" and harden a little bit more than I like.
1 TBL Vanilla extract. Add to butter/sugar bowl.
It's easiest if you combine the butter and vanilla with the sugar first, in one bowl. (use your hands - faster)
In 2nd, larger bowl, measure out the flour (and optional leavening) and mix well using hands or a wire whisk/whip. (no sifting needed).
Combine butter/sugar (and vanilla) mixture into the flour mixture.
Grease a long sheet pan (13X9 inches) that has a rim around it and pat handfuls of this crumbly batter into that pan. Lightly pat the dough to get it as evenly as possible.
Bake 350 for 15 minutes, then lower temp to 300 and bake additional 30 minutes. Remove pan from oven and immediately cut into bars. If you wait until the dough cools down, it will crumble when you try to cut it. Do it while it's still hot. If you forget, return the pan to the oven for a few minutes to reheat the Shortbread and then cut it.
VERY, VERY IMPORTANT: DO NOT LET DOUGH BROWN AT ALL. Place your oven rack on the 3rd rung above the oven floor so that neither the top nor the bottom of the cookie bars will brown. The MOST your dough should become is a light gold. Mine was still almost white when the time was up and I took it out. Browned bars will harden too much as they cool down and the browning destroys the delicate buttery flavor.
This is a great "base" recipe. Here are some other suggestions for making it a bit differently:
Add chocolate chips to it (put them in it or, instead, melt them on top of it) and the flavor should rival any chocolate chip cookie recipe in your file.
At Christmastime use Peppermint extract instead of vanilla and frost the bars with chocolate frosting.
At Easter, use Almond extract and frost with Buttercream or a Cream Cheese frosting.
In place of fruitcake for Christmas, add chopped dried cherries, dried peaches, dried apricots, dried cranberries, dried blueberries/walnuts, pecans to this recipe.
How much should you add? The dough's texture should tell you. Mix in some chocolate chips, dried fruits or nuts and then pick up a handful of dough and lightly squeeze it. Does it still hold it's shape? Good. Add some more "additions". Squeeze some dough again. Not holding it's shape so easily? A bit crumbly now? Add nothing more to the dough and bake it.
Remember to slice while hot when it comes immediately out of the oven. The bars taste better the 2nd and 3rd days and so forth. If you can resist temptation, bake them, slice them, cool them, place them into a large container and leave them to sit for a few days before you eat them.
Nah....I couldn't do it, either. :-)