Place the chicken cubes in a gallon-size zip-top bag and pour in the buttermilk; seal and toss to coat evenly. Refrigerate briefly while you prepare the coating if desired.
Preheat the oven to 425°F (220°C). Line one or two baking sheets with parchment paper to avoid crowding the chicken.
In a medium bowl, combine the finely crushed corn flakes, flour, 2 tablespoons Cajun seasoning, shredded cheddar, and a pinch of black pepper; stir until evenly mixed.
Working one piece at a time, remove chicken from the buttermilk and dredge in the corn flake mixture, pressing gently so the crumbs adhere; place coated pieces on the prepared baking sheet with space between them.
Lightly brush the tops of the coated chicken pieces with olive oil. Bake at 425°F for 15–20 minutes, or until the chicken is cooked through and the coating is golden and crisp.
Meanwhile, make the creamy honey mustard by combining Dijon mustard, honey, sour cream (or Greek yogurt or mayo), 2 tablespoons olive oil, lemon juice, 1 tablespoon Cajun seasoning, 1/4 teaspoon cayenne, and a pinch of kosher salt and black pepper in a jar or bowl; whisk or shake until smooth.
Remove the chicken from the oven and, if desired, finish with flaky salt and chopped fresh chives. Serve hot with the creamy honey mustard for dipping.