Place the flour and baking powder in a bowl. Add the butter and rub together with your fingers for about 5 minutes.
Make a well in the centre, crack in the egg, then add the salt, water and sugar. Knead everything by hand until a dough forms. Shape into a ball.
Grease the cake tin(s) with a little butter. Roll out the dough thinly and place in the tin. Prick the base with a fork. If any dough remains, shape into a ball and roll out again for extra small tins.
Preheat the oven to 180°C (top/bottom heat).
Wash and pit the plums, then cut them into halves or quarters. Arrange them evenly over the pastry.
Bake for 40-45 minutes. Remove from the oven and leave to cool.
Whip the cream with the sugar using a hand mixer until stiff.