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1
Whisk together the egg white, vinegar and 3 tablespoons water in a small bowl; set aside.
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2
Refrigerate the egg yolk in a small bowl, covered, until ready to use.
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3
Pulse the flour, salt, mustard, paprika and turmeric in a food processor to combine.
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4
Add the butter, Cheddar and Parmesan, and pulse until the butter is completely broken up.
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5
Pour in the egg white mixture, and pulse until the dough comes together in a ball.
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6
(It's OK if it's a little wet.)
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7
Place the dough on a large piece of plastic wrap, and pat it into a 1/2-inch-thick square.
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8
Wrap it up, and refrigerate to chill and firm up the dough, about 1 hour.
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9
Position 2 oven racks at the top and bottom thirds of the oven, and preheat to 350 degrees F. Line 2 baking sheets with parchment.
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10
Roll the dough out on a large piece of parchment (to keep it from sticking to the counter) into a 10-inch square about 1/8 inch thick.
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11
(Dust the dough with flour if you find it too sticky.)
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12
Trim the edges to straighten the square, then cut it into 1-inch squares (a pizza cutter works great).
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13
Whisk 2 teaspoons water into the egg yolk.
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14
Brush the mixture over the tops of each cracker.
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15
Sprinkle each with the tiniest pinch of salt.
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16
Bake the crackers, 2 baking sheets at a time, until they are deep golden brown on the bottom, about 20 minutes, rotating the sheets about halfway through.
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17
Let the crackers cool on the baking sheet for a few minutes, then transfer to a cooling rack to cool completely.
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18
Store the crackers at room temperature in an airtight container for up to 2 days, or freeze them for up to 1 week.
