Sprinkle plums with Armagnac and leave to soak for at least 6 hours, preferably overnight.
Heat milk, sugar and 60 g butter while stirring until butter and sugar have dissolved. Leave to cool slightly.
Preheat oven (electric cooker: 175°C/circulating air: 150°C/gas: level 2). Grease an ovenproof casserole dish (approx. 23 x 30 cm; approx. 7 cm high). Mix eggs and 125 g flour with the whisks of the mixer.
Slowly stir in the milk mixture. Pour the dough into the mould.
Dab plums dry if necessary. Dust with about 1 tbsp. flour and spread on the dough. Bake in a hot oven for about 45 minutes. Let them cool down. Dust with icing sugar.