For the chicken: In a small bowl, mix together the buttermilk, hot sauce, kosher salt, granulated garlic and granulated onion. Place the chicken into a re-sealable bag and pour the buttermilk marinade over the chicken. Marinate for at least 2 hours, or overnight, in the fridge.
Preheat the oven to 200 degrees F and have 2 baking sheets with wire racks set inside. Preheat a waffle maker according to the manufacturer's instructions. In a large cast-iron skillet, bring an inch of oil to 350 degrees F.
In a baking dish, add the flour and season it with the paprika and some salt and pepper. In another baking dish, add the panko and season it with the ancho chile powder and some salt and pepper. In another dish, beat the eggs.
Remove the chicken from the marinade, allowing the excess buttermilk to drip off. Then dredge the chicken first in the flour, then in the eggs and finally coat the chicken in the panko.
Fry the chicken in batches, being careful not to overcrowd the pan, until each side of the chicken is golden brown and the interior reads 160 degrees F on an instant-read thermometer, 3 to 4 minutes per side. Remove the fried chicken to one of the prepared baking sheets and place into the warm oven to stay warm and crispy while you make the waffles.
For the spicy maple butter: Mix the butter, syrup, hot sauce and cayenne in a bowl.
For the waffles: Whisk the oil, vanilla and eggs in a medium bowl until combined. Add the buttermilk and seltzer.
In another bowl, whisk the flour, sugar, table salt and baking soda to combine. Add the dry ingredients to the wet ingredients and whisk until smooth. Stir in the scallions.
Spray the waffle maker with some nonstick cooking spray. Pour the batter in the waffle iron and cook until golden. Set aside in a warm oven on the other prepared baking sheet until ready to serve.
For the sandwich build: Spread each waffle with a generous portion of the maple butter. Place some tomatillo slices on a waffle and then some of the hot crispy chicken. Top with the other waffle to seal the deal.