Arrange the chicken pieces in a single layer in a large ovenproof dish.
Mix together the butter, mustard, lemon juice, brown sugar, and paprika in a bowl and season with salt and pepper to taste. Brush the mixture over the upper surfaces of the chicken pieces and bake in the oven at 400F for 15 minutes.
Remove the dish from the oven and carefully turn over the chicken pieces. Coat the upper surfaces of the chicken with the remaining mustard mixture. Sprinkle the chicken pieces with poppy seeds and return to the oven for a further 15 minutes.