In a large bowl, mix the flour, salt and baking soda. Add the yeast mixture and stir vigorously with a wooden spoon to blend. If the dough is sticky, add sprinkles of flour until a soft dough forms.
Knead the dough until it is soft and has an elastic consistency, about 5 minutes. Add sprinkles of flour to control the stickiness. If using a mixer or a food processor, the dough will form a soft ball around the revolving dough hook and clean the sides of the bowl. Add flour, if necessary, to firm up the dough.
Drop the dough into a buttered bowl, cover with plastic wrap and place in the refrigerator to rest for at least 1 hour and up to 18 hours (the longer the better.)
Arrange the rack in the middle of the oven and preheat to 425 degrees F. Line a baking sheet with parchment paper.
With a heavy rolling pin, roll the dough into a rectangle about 18 by 6 inches and no thicker than 1/8-inch. Fold the dough from the short ends, brushing off the excess flour, to make 3 layers for extra flaky crackers. Roll again using the rolling pin.
Prick the dough with the tines of a fork to help cook evenly. Evenly cut the dough along the edge of a ruler or yardstick with a pizza or cookie cutter into desired shapes.
Place the crackers close together on the prepared baking sheet. Sprinkle lightly with salt from 12-inches above the crackers to distribute evenly.
Bake until lightly browned and crisp, 15 to 20 minutes, depending on the thickness of the crackers. Check the crackers several times during the baking period to make certain those on the outer edge of the baking sheet are not getting too brown. If so, switch the ones on the outside with the ones in the middle.
Remove from the oven and brush the crackers with melted butter. Let cool on a metal rack.
Sesame-Onion Crackers: Add to the dough, with the dry ingredients, 4 teaspoons each sesame seeds and grated onion.
Herb Crackers: Add to the dough, with the dry ingredients, 4 teaspoons each chopped fresh parsley and chives and 1/2 teaspoon dried dill weed.
Poppy Seed Crackers: Add to the dough, with the dry ingredients, 2 teaspoons poppy seeds.