In a small bowl dissolve the yeast and ¼ teaspoon sugar in the milk. Let rise. In another large bowl combine the flour, sugar and salt. Add the egg, lemon zest and vanilla sugar. Gradually add the milk/yeast mixture and melted butter. Knead until the dough becomes smooth and pliable. Cover with plastic wrap and let rest in a warm place for 2 hours.

Cover the bottom of a 32-cm pie dish with parchment paper. Preheat oven (175°C/350°F).

Roll out the dough on a slightly floured surface and transfer to the prepared sheet. Grease the surface of the dough with the one tablespoon melted butter and arrange the plums on it.

Prepare crumb: Mix the flour, sugar and cinnamon in a mixing bowl and gradually add the lukewarm melted butter. Mix with a fork until clumps form. Spread evenly over the plums. Bake on second lower rack for about 40 minutes. Remove to the middle rack for about 2-3 minutes to brown crumb a little