Make the chicken: Whisk the buttermilk, hot sauce, granulated garlic, granulated onion and 3/4 teaspoon kosher salt in a small bowl. Place the chicken in a resealable plastic bag and pour in the buttermilk marinade. Refrigerate at least 2 hours or overnight.
Preheat the oven to 200 degrees F and have 2 baking sheets with wire racks set on top. Preheat a waffle iron. Fill a large cast-iron skillet with 1 inch vegetable oil and heat to 350 degrees F on a deep-fry thermometer.
Put the flour in a baking dish and season it with the paprika and salt and pepper. Put the panko in another baking dish and season it with the chile powder and salt and pepper. Beat the eggs in another dish.
Remove the chicken from the marinade, allowing the excess to drip off. Dredge the chicken first in the flour, then in the eggs and finally coat with the panko. Fry the chicken in batches, being careful not to overcrowd the skillet, until golden brown and an instant-read thermometer inserted into the center reads 160 degrees F, 3 to 4 minutes per side. Remove to one of the prepared baking sheets and keep warm in the oven.
Make the spicy maple butter: Mix the butter, maple syrup, hot sauce and cayenne in a bowl.
Make the waffles: Whisk the vegetable oil, vanilla and eggs in a medium bowl until combined. Whisk in the buttermilk and seltzer. In another bowl, whisk the flour, sugar, baking soda and fine salt; add to the wet ingredients and whisk until smooth. Stir in the scallions.
Coat the waffle iron with cooking spray. Pour in some batter and cook until golden, 4 to 6 minutes. Place the waffles on the other baking sheet and keep warm in the oven while you make the rest.
For the sandwich build: Spread each waffle with a generous portion of the maple butter. Place some tomato slices on a waffle and then some of the crispy chicken. Top with another waffle to seal the deal.