Preheat the oven to 425 ° and line a sheet pan, that will hold all the chicken pieces without touching each other, with aluminum foil.
Melt the 2 teaspoons of butter in a small skillet over medium heat. Stir in the panko and breadcrumbs, toast for a few minutes, stirring occasionally, until golden brown. Remove from the heat.
Mash in the remaining 3 tablespoons of butter into the mixture.
Season the chicken lightly with salt and pepper, then brush or slather the mustard mixture all over, then roll the entire piece of chicken into the breadcrumb mixture. Place the chicken on the baking sheet.
Roast for 25 minutes, or until the chicken is cooked through; an instant-read thermometer inserted into the thickest part of the chicken should register 165 degrees. Depending on the size of the chicken pieces it may take less or more time to cook.