Preheat oven to 350˚ F. Cut butter into flour until it resembles coarse sand or cornmeal.
Add brown sugar, cinnamon, baking soda, cream of tartar, and egg and begin to mix. Add enough hot water to the mixture to make dough that can be rolled like pastry. If the dough seems to want to fall apart, you may want to work it and knead it just 2-3 times with your hands, either in the bowl or on a board with a little flour, just until it sticks together to form a ball.
On a large ungreased or parchment-lined baking sheet such as a 13 x 18” jelly roll pan or baking sheet, roll or pat dough into 1/8 to 1/4-inch thick rectangle. Lightly dust with flour if needed. If the dough is too sticky to roll with a rolling pin, it also works to pat and press the dough out with your fingers dipped in flour.
Using a pizza cutter, a wheel or a knife, cut dough into 3-inch squares. (The lines will bake back together, but don’t worry; you’ll be able to break them apart when cool.) Prick squares with a fork at regular intervals. Optional: Sprinkle with white sugar for a little sweetness on the top.
Bake 15 to 20 minutes, just until barely browned at edges. Cool on rack. Optional: If you didn't sprinkle with sugar before, you may brush tops of warm cookies with butter and sprinkle with sugar.
Notes: Graham flour, because it’s less adulterated than refined flour and still includes the wheat germ, will go bad more quickly than other more processed flours. (Natural oils in the germ, which most mills leave out of the finished product, can go rancid at room temp). If you don’t think you’ll use up your bag within a month or two, think about keeping it in the fridge or freezer.