Activate Yeast: Combine warm milk, yeast, and 2 tbsp of sugar; let sit for 5-10 minutes until frothy.
Prepare Dough: Mix flour and salt, then add yeast mixture, melted butter, rest of the sugar and egg. Knead until smooth.
First Rise: Let dough rise in a covered bowl for 1 hour.
Preheat Oven: Preheat oven to 350°F (180°C).
Prepare Topping: Slice plums; make streusel by mixing sugar, flour, and butter.
Assemble Cake: Spread dough on a 13 x 18 inches (standard half sheet) baking sheet, top with plums and streusel.
Bake: Bake for 30-35 minutes until golden.
