Warm the milk lukewarm. Crumble yeast into it and dissolve it while stirring. Put 300 g flour, 75 g sugar, 1 packet vanilla sugar, 1 pinch of salt, 1 egg and 75 g butter in a mixing bowl. Add yeast milk and knead everything with the dough hooks of the mixer to a smooth dough.
Cover and leave to rise in a warm place for about 40 minutes.
Wash the plums, dab dry, halve and stone them. For the crumbles, knead 150 g flour, 100 g sugar, 1 packet vanilla sugar, 1 level teaspoon cinnamon, 1 egg yolk and 80 g butter with your hands to form crumbles.
Line two baking trays with baking paper. Knead the yeast dough once again on a floured work surface. Form into a roll and cut into 8-10 equally sized pieces. Form pieces with floured hands into round flat cakes (approx. 10 cm Ø each).
Place them at a distance on the two sheets. Cover and leave to rise in a warm place for about 20 minutes.
Preheat oven (electric cooker: 200 °C/circulating air: 175 °C/gas: see manufacturer). Cover the dough cakes evenly with the plums. Whisk 1 egg yolk and 1 tbsp. water, brush the edges of the dough with it. Spread the crumbles evenly on the plums.
Bake the crumbled snails in the hot oven for about 20 minutes each. Take them out of the oven and let them cool down on a cake rack. Dust the crumble snails with icing sugar shortly before serving.