Heat the milk to a lukewarm temperature. Stir in the sugar and yeast and leave to rest for 10 minutes. The yeast should activate and you will be able to see bubbles appearing on the surface. If the bubbles do not appear, the milk might have been too hot and damaged the yeast.
In a separate bowl, add the flour and salt. Pour over the yeast mixture and add the butter and egg. With a hand or stand mixer, knead the dough for about 10 minutes until you achieve a smooth, not too sticky dough.
Turn off the oven. Place the dough in a bowl, and cover it with a moist tea towel. Leave to rise for about an hour in the warmed-up oven. After an hour, the dough should have doubled in size.
Make the Streusel
While your dough rises, melt the butter in a small saucepan. Remove from the heat and stir in flour, sugar, cinnamon, and the vanilla extract or sugar until you get fine crumbs. Refrigiate until needed
butter to grease the pan , 1 ¼ cup all-purpose flour, ⅓ cup granulated sugar, ⅓ cup butter, 2 tsp ground cinnamon, 1 tsp vanilla extract
Wash and drain the plums. Make crosswise cuts at both ends, then slice along one cut to open and remove the stone.
2 lb Italian prune plums
Preheat your oven to 200°C/392°F top to bottom heat.
Assemble the Cake
Knead the risen dough briefly on a floured surface, then roll out to fit your baking tray. Grease the tray or line with parchment paper. Press the dough into the pan.
butter to grease the pan
Arrange plum halves in overlapping rows on the dough. Sprinkle streusel on top.