Combine all ingredients except for the butter in a sauce pot over medium heat on the stove. When it is simmering, add in butter and stir until melted. Take off heat and set aside. Keep warm. For blue cheese sauce:
Mix together flour, cornstarch, smoked paprika, cayenne pepper, kosher salt, and freshly ground pepper in a shallow bowl. Put beaten eggs and panko breadcrumbs into 2 separate shallow bowls or mixing bowls. Using a kitchen knife and cutting board, butterfly chicken breasts into 8 separate pieces. Season with kosher salt and freshly ground black pepper.
Dredge chicken in flour mixture and shake off excess. Dip into beaten eggs allowing excess to drip off, then coat the chicken in the panko, pressing to help adhere. Place chicken on a baking sheet lined with parchment. Repeat with remaining chicken pieces.
Add 1½ cups of oil in a large skillet or deep saute pan. Heat until oil is glistening. Add chicken and fry over medium heat, turning once, until golden brown on each side. Approximately 4-5 minutes per side. Drain on paper towels.
Using kitchen tongs, put crispy chicken into buffalo sauce and coat thoroughly. Set on rack over a baking sheet.
Discard oil in skillet and carefully wipe out with paper towels. Put skillet back on stovetop over medium heat with 2 tablespoons of oil. While skillet is heating, build sandwich in the order from the bottom up: sliced bread, nacho cheese, buffalo chicken, sliced red onion, cilantro, sliced bread.
Placed sandwich on hot skillet. Grill until golden brown. Turn sandwich over and grill until golden brown.