Preheat oven to 200°C/ gas mark 6. Combine the chicken and buttermilk in a shallow dish. Cover and chill for 15 mins.
Spray a baking sheet with cooking spray. Put the cereal in a sealed plastic bag and crush with a rolling pin. Transfer the crumbs to a shallow dish. Season the chicken with the salt and a few grinds of pepper. Dip each piece of chicken in the cereal to fully coat and arrange on the baking sheet. Bake until cooked through, about 8 mins. Leave the chicken on the baking sheet to cool slightly. It will become crispier.
Serve with the mustard sauce.
For the honey mustard sauce: In a small bowl, stir together the mustard and mayonnaise until smooth. Stir in the honey.