for the Filling
3 1/2 lbs damson/italian/fresh prune plums
For the Topping
1/2 cup sugar
1/4 cup all-purpose flour
1/4 cup butter, cubed
2 teaspoons ground cinnamon
In the mixing bowl of your stand mixer, mix together the yeast, 1 cup of the flour, half the sugar and the milk. At first, this yeast-mixture should be still fluid. Cover and let rise in warm place for 20 minutes. and it will start to foam. Add remaining sugar, melted butter, egg and a good pinch of salt and mix with yeast mixture and flour. Mix with the mixer until the dough pulls away from the sides of the bowl. Turn onto the counter and knead with floured hands for 8-10 minutes. Use some additional flour, if dough is too sticky. At the end you should have a smooth dough. Cover in a greased bowl and let rise in warm place for about1 hour, or until dough has doubled.
Meanwhile prepare the plums. Cut lengthwise and pit them. Cut the still connected plum halves once again lengthwise. You should have a depitted plum which you can easily flatten.
Grease a standard sized rectangular sheet pan (11 3/4 x 15 inches). Remove yeast dough from bowl, knead once or twice and roll out on baking tray using a floured rolling pin. Along the sides of the baking tray form a border with yeast dough. Place plums on dough in closely rows and press slightly into dough. Let rise again for 15 minutes.
Preheat oven to 425F.
Mix together topping ingredients (sugar, flour, cinnamon and sugar) with your fingers or a fork. Sprinkle evenly over the plums in a light layer.
Bake the plum cake in the middle of the oven for 35-40 minutes.