Slice the tofu into four large pieces by halving it lengthwise and then crosswise.
Dry the tofu very well by pressing gently but thoroughly between multiple layers of paper towels. Keep going until it hardly yields any liquid when you press.
In a shallow bowl or baking dish, whisk together the soy milk and vinegar. Add half the onion powder, half the garlic powder, half the paprika, half the salt, half the cayenne, and half the pepper. Whisk well.
Add the tofu slices. Marinate at room temperature for 15 minutes, then flip once and marinate 15 minutes more.
On a large plate, whisk together the flour, cornmeal, and remaining onion powder, garlic powder, paprika, salt, cayenne, and pepper.
One by one, remove tofu slices from marinade, letting any excess drip off. Dredge thoroughly in flour mixture.
Pour oil to a depth of about ½ inch in a 12-inch cast iron skillet. Heat over medium-high until thin and shimmering. If you are using a thermometer or temperature-controlled burner, try to keep the oil temperature at 350°F.
Dredge each tofu piece in the flour mixture again, and place into skillet.
Fry until crisp and golden, flipping once, about three minutes per side. Drain on a wire rack set over a baking sheet.
To assemble the sandwiches, place one piece of tofu onto each bun and top with pickles, lettuce, mayo, and sriracha.