Stir the flour, cornmeal, cornstarch, salt, mustard and cayenne together in a medium bowl. With a pastry blender or two forks, cut in the butter, until the pieces are the size of blueberries. Pour the cold liquid over the flour, stirring with a fork until the mixture holds together.

(Alternatively, place all of the dry ingredients in a food processor work bowl. Pulse to combine. Add the butter and pulse until the pieces are the size of blueberries. Pour the liquid into the food processor all at once. Pulse until the dough forms a ball, but do not over-process.)

Whichever method you choose, divide in three equal portions.

On a piece of parchment or waxed paper, form each portion of dough into a log, 12 inches long, about 1 1/4-inch round. Wrap the parchment around the dough and use it to round out the log. Refrigerate for 2 hours or freeze for 45 minutes, until firm enough to slice.

Preheat the oven to 400°F. Line large baking sheets with silicone baking mats or parchment paper. Slice about 1/8-inch thick and arrange on baking sheets 1/2 inch apart.

Bake 10 minutes, until the coins are barely golden around the edges.