Preheat oven (electric cooker: 200 °C/circulating air: 175 °C/gas: see manufacturer). Place paper cups in a mini muffin baking tray (24 troughs). For the crumbles, mix flour, sugar and butter in pieces with your fingers to form crumbs.
For the muffins, mix flour, vanilla sugar, sugar, 1 pinch of salt and baking powder. Mix egg, yoghurt and oil. Using the whisks of the mixer, stir briefly into the flour mixture until all ingredients are moist.
Fold in the frozen berries. Spread the dough with teaspoons into the moulds. Sprinkle with crumbles. Bake in a hot oven for about 15 minutes (in "normal" muffin size about 25 minutes).
Let the muffins cool down for about 10 minutes. Lift them out of the troughs and let them cool down.