Adjust racks to the top and bottom third of the oven. Preheat oven to 350 degrees F. Line two baking sheets with parchment paper or silicone liners; set aside.

In the bowl of a food processor fitted with the dough blade, process the cheese, butter, flours, onion powder, salt and 2 Tablespoons of the milk until a dough is formed. Add up to ½ Tablespoon more of milk if the dough is too dry and crumbly.

Transfer the dough to a pastry mat or a clean floured surface. Roll to a large rectangle until the thickness of the dough is about 1/16th of an inch. Using a pastry wheel or a pizza cutter, cut the dough into 1x1-inch squares, or ½x1/2-inch squares (really, it doesn't matter too much what size you choose) but the baking times may vary. Reserve any scraps to repeat and roll out and cut again.

Using a thin spatula, transfer the dough squares onto the baking sheets. Lightly brush milk onto the tops of each square with a pastry brush, then sprinkle with Kosher salt. Prick the center of each square with the blunt end of a bamboo skewer.

Bake the two baking sheets of crackers for 12 to 18 minutes, depending on the size of your crackers, until they are crisp and slightly golden. Rotate the pans from top to bottom and back to front halfway through baking.

Remove from the oven and set the baking sheets on wire racks. Allow to cool completely.