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To make the dough: Combine the buttermilk and tahini in a small bowl and set aside.
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In the bowl of a stand mixer fitted with a paddle, combine the flours, sugar, salt, cumin, allspice, and butter. Mix until the butter is in very small pieces.
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Add the buttermilk mixture all at once and mix until a wet dough forms.
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Line a baking sheet with parchment. Divide the dough in quarters; work with one quarter at a time, keeping the rest of the portions in the refrigerator.
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On a floured work surface, roll one portion of the dough into a rectangle about 12" x 13" and 1/16" thick. Sprinkle the parchment on the baking sheet with 1 tablespoon of the cornmeal.
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Transfer the rolled dough to the prepared pan, place another sheet of parchment on top, and sprinkle another tablespoon of cornmeal on it.
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Repeat with the remaining dough pieces, placing them on cornmeal-sprinkled parchment and stacking them on the baking sheet. When all are done, wrap the baking sheet with plastic and refrigerate for 4 hours, or as long as overnight.
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Preheat the oven to 350°F.
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To bake: Take one sheet of parchment with dough on it and place it on an empty baking sheet. Prick the dough all over with a fork. Brush it lightly with water and sprinkle with 1/4 teaspoon of the kosher salt from the topping ingredients and 1 1/2 teaspoons of the sesame seeds. Use a ruler and a pizza cutter to cut the dough into roughly 3" squares.
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Bake the crackers for 6 minutes, then rotate the pan and bake for an additional 6 to 7 minutes, until lightly browned. Remove from the oven and cool on a rack before breaking into individual crackers. Repeat with the remaining sheets of dough.
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Store crackers cool and dry in an airtight container for up to 2 weeks.