1) Whisk together the flour, yeast, salt, baking soda, cream of tartar, dough improver, and sugar. Set it aside.
2) Put the water, butter, and oil in a microwave-safe cup, or in a saucepan. Heat gently just to melt the butter. Remove from the heat, and cool to 120°F-130°F. If you don't have a thermometer, this will feel hotter than lukewarm, but not at all uncomfortably hot; it'll be cooler than your hottest tap water.
3) Add the liquid ingredients to the dry ingredients. Beat at medium, then high speed for a total of about 90 seconds, to make a soft dough.
4) Place the dough in a lightly greased bowl, cover it, and refrigerate overnight, or for up to 18 hours. It won't rise much; the bowl can be small.
5) Remove the dough from the refrigerator, and allow it to rest for about 15 minutes. Preheat the oven to 425°F.
6) Lightly flour a work surface (a silicone rolling mat works well here), and remove the dough from its rising bowl. It won't feel like normal yeast dough; it'll be more clay-like. Shape the dough into a 3" x 5" rectangular block; pre-shaping it like this will help you roll it out evenly. Roll it into a rough 13" x 15" rectangle; it'll be quite thin. Be sure to keep the rolling surface well-floured, to avoid sticking.
7) Starting with a shorter side, fold the dough in three like a business letter.
8) Roll it out again, this time to an 11" x 19" rectangle, or thereabouts. The dough will shrink when you stop rolling it; your goal is to end up with a rectangle that's about 10" x 18".
7) Sprinkle the dough with your choice of salt — we like an herbed or smoked salt — and gently press it in with the rolling pin.
8) Using a rolling pizza wheel (easiest) or a baker's bench knife, cut the dough into 2" squares. Note: If you're using a silicone mat, cut very carefully - you don't want to damage the mat. We like to use an acrylic-blade pizza wheel.
9) Transfer the crackers to two lightly greased or parchment-lined baking sheets; you can put them fairly close together, as they'll shrink as they bake, rather than spread. Prick each cracker once or twice with the tines of a fork.
10) Bake the crackers for about 10 minutes, till they're a very light golden brown. Watch them carefully towards the end of the baking time; they can darken very quickly.
11) Turn off the oven, and open the door completely. Leave the crackers on the oven rack; they're going to cool down right in the cooling oven, in order to preserve their crispness. Keep your eye on them for the first couple of minutes; if for some reason your oven isn't cooling off quickly, and the crackers are continuing to brown, pull the rack out partway.
12) When the crackers are completely cool, remove them from the oven, and wrap airtight, to preserve their crispness.
Yield: about 45 crackers.