Put flour in a bowl, mix well with 40 g sugar and press a depression in the middle. Warm buttermilk lukewarm, mix well with yeast and honey. Pour yeast milk into the hollow, mix with some flour from the edge to a thin paste.
Cover the pre-dough and let it rise for about 15 minutes.
Defrost the berries in a sieve. Melt butter. Whisk the egg. Add butter, egg and rum to the pre-dough and knead to a smooth dough. Cover and leave to rise in a warm place for about 45 minutes.
Line an ovenproof casserole dish (approx. 26 cm Ø) with parchment if necessary. Stir pudding powder, 3 tbsp. sugar and 6 tbsp. milk until smooth. Bring the remaining milk to the boil in a small pot. Stir the pudding powder into the boiling milk, let it simmer for about 1 minute, remove from the heat.
Pour the pudding into the mould, set aside.
Briefly knead the dough on a lightly floured work surface and form into 5-6 small rolls. Spread the rolls into the form. Sprinkle berries in between. Bake in a preheated oven (electric cooker: 175 °C/ circulating air: 150 °C/ gas: see
manufacturer) bake for about 20 minutes. Remove the mould from the oven, let it cool slightly and dust with icing sugar.