Pre-heat the oven to 160ºC (320ºF). Prepare a small spring-form pan (mine is 6" or 18cm) by greasing or lining with baking paper.
Beat the sugar and butter together until it is pale and fluffy. Add in one egg at a time and continue to beat until well incorporated. Finally, add the vanilla extract.
In a separate bowl, mix the baking powder and flour and fold into the wet ingredients.
Pour the thick batter into the prepared tin and level it off. Place the plums skin side down in the batter and bake for around 30 minutes. Aim for a cake that is still a little soft in the middle.
While the cake is baking, prepare the topping. Melt the butter in a small pan. Remove from the heat and add the other ingredients, giving it a stir until everything is well combined.
When ready, remove the cake from the oven and set the temperature to 180ºC (350ºF). Sprinkle the topping over the cake and spread it so that there is an even coverage. Return the cake to the oven and bake for a further 20-25 minutes, until the cake is nice and golden. Serve warm with cream or custard. Enjoy!