Season the chicken breast fillets with salt, pepper, and curry powder. Place in a baking dish.
Beat the egg whites until stiff peaks form.
Mix the grainy mustard, medium-hot mustard, breadcrumbs, and a pinch of salt together.
Fold the mustard mixture into the beaten egg whites until combined.
Spread the mustard-egg mixture evenly over the top of each chicken breast fillet.
Top each with the butter flakes.
Bake in a preheated oven at 200°C (392°F) with top and bottom heat for 20-25 minutes, until the chicken reaches an internal temperature of 74°C (165°F).