Prep the plums: Wash, halve, and pit the plums. Slice them into quarters if large. Set aside.
Make the dough: In a bowl, beat butter and sugar until creamy. Add the egg and vanilla. Mix in flour, baking powder, and salt until smooth. Spread the dough evenly into a greased 20 cm (8-inch) baking pan.
Arrange the plums: Place the plum slices on top in rows, slightly overlapping for a traditional look. Sprinkle lightly with cinnamon.
Make the streusel (optional): Rub flour, sugar, and cold butter together with your fingers until crumbly. Sprinkle over the plums.
Bake: Bake in a preheated oven at 180°C (350°F) for 35–40 minutes, or until golden and the plums are bubbling slightly.
Serve: Let cool a little, then serve warm. Pair with whipped cream or vanilla ice cream.