Peel, wash and halve the potatoes. Boil potatoes in boiling salted water for about 25 minutes until soft. Wash, clean and cut the celery into pieces. Peel the pearl onions. Wash the guinea fowl breasts, dab dry and season with salt and pepper.
Heat the oil in a frying pan. Fry the guinea fowl breasts in it first from the skin side for about 3 minutes until crispy. Add celery, onions, 1 tbsp. butter and chestnuts and fry briefly. Turn the guinea fowls. Add spices and deglaze with champagne, stock and Madeira, bring to the boil.
Braise in the preheated oven (electric cooker: 175 °C/ convection oven: 150 °C/ gas: stage 2) for approx. 10 minutes. Wash the marjoram, shake dry and chop the leaves from 6 stalks. Switch off the oven.
Remove guinea fowl from the roaster and keep warm in the oven. Pass remaining liquid and vegetables through a sieve, put vegetables aside. Let the liquid boil down to just under 200 ml. Season to taste with salt, pepper and a little sugar.
Boil up the milk. Drain the potatoes. Add milk to the potatoes and mash. Season the puree with salt, pepper and nutmeg. Cut 50 g butter into cubes. Add the butter cubes to the sauce and foam up with the blender.
Add the vegetables and chopped marjoram, warm briefly (do not boil). Halve guinea fowl breasts. Serve with mashed potatoes and vegetables. Garnish with marjoram.