Place the cubed chicken in a gallon-size zip-top bag and pour in the buttermilk; toss or shake to coat evenly. Refrigerate briefly if desired for up to 30 minutes.
Preheat the oven to 425°F (220°C) and line a baking sheet with parchment paper.
In a medium bowl, combine the finely crushed corn flakes, flour, 2 tablespoons Cajun seasoning, shredded cheddar, and a pinch of black pepper; stir to combine.
Working one piece at a time, remove chicken from the buttermilk and dredge in the corn flake mixture, pressing gently so crumbs adhere. Arrange pieces on the prepared baking sheet without crowding; use a second sheet if needed.
Lightly brush the coated chicken pieces with olive oil and bake for 15–20 minutes at 425°F, until cooked through and golden.
While the chicken bakes, make the honey mustard: combine Dijon mustard, honey, sour cream (or Greek yogurt or mayo), 2 tablespoons olive oil, juice of 1/2 lemon, 1 tablespoon Cajun seasoning, 1/4 teaspoon cayenne, and salt and black pepper to taste; whisk or shake in a jar until smooth.
Serve the popcorn chicken warm with the creamy honey mustard for dipping, and sprinkle flaky salt or fresh chives if desired.
