Grease a springform pan (26 cm Ø) and dust with flour. Wash, halve and stone the plums.
Preheat oven (electric cooker: 175 °C/circulating air: 150 °C/gas: see manufacturer). For the dough, whisk 200 g butter, 200 g sugar and 1 pinch of salt until creamy. Stir in 4 eggs one after the other.
Stir 250 g flour into the egg-butter mixture by the spoonful. Stir in rum. Fill the dough into the springform pan. Spread the plums on the dough, pressing them in lightly.
For the cream mixture, stir cream, sauce powder and 50 g sugar until smooth. Stir in 2 eggs. Pour the glaze evenly over the plums. Bake in a hot oven for about 45 minutes.
Remove the cake and let it cool down. Remove from the mould and dust with icing sugar. Whipped cream tastes good with it.