For the dough, mix flour with baking powder and oatmeal. Beat butter, sugar and egg until creamy and knead with the flour mixture. Halve the dough. Roll out each half into a rectangle (16 x 24 cm).
Cut out 6 squares of 8 x 8 cm per half of the dough and place each in a muffin tin.
Wash and stone the plums, then quarter. Place 4 wedges of plum on the tips of the dough in each mold. Mix eggs, sugar, cinnamon and sour cream and distribute in the molds.
Mix the flour with the oat flakes, cane sugar, vanilla sugar and cinnamon and add the butter. Process everything into crumbles in the desired size. Sprinkle the crumble on the sour cream filling between the plums.
Bake at 180 ° C for about 35-40 minutes. Let the muffins stand in the mold for 10 minutes, release and let cool on a wire rack.
If you use plums, please cut the plums into eighths, not quarters. You only need 6, not 12, plums.