In a large bowl, combine the chicken, jalapeño juice, buttermilk and hot sauce. Cover with plastic wrap and chill for at least 1 hour, more if you have the time.
In a shallow bowl, whisk together the flour, salt, pepper, onion, garlic, and chili powder.
To dredge the chicken, remove each piece from the buttermilk mixture and dip into the seasoned flour. Dip once again into the buttermilk and once more into the dry ingredients. Place the dredged chicken on a wire rack set over a baking sheet and continue coating the remaining chicken pieces. Allow the coated chicken to rest while you heat up the oil. This will prevent the coating from falling off during frying.
Pour the oil into a large pot and heat up to 365 degrees F. Carefully lower the chicken into the hot oil, cooking it in batches to not overcrowd the pan. Fry until golden brown and the chicken reaches 165 degrees, about 4 to 6 minutes. Using a slotted spoon, remove the chicken from the oven and place on a clean wire rack, set over a baking sheet.
Set a large pan or griddle over medium high heat, melt the butter and toast the buns, cut side down.
To assemble the sandwiches, spread the buns with mayonnaise and place a chicken on each bottom bun. Top with lettuce and tomato and the top bun. Serve immediately.