Preheat the oven to 180 C, 160 Fan Gas 4. Line a 21cm springform cake tin with baking paper.
Beat together the sugar and butter for the cake until pale and fluffy. Add one egg at a time, beating to incorporate.
In another bowl mix together the baking powder, salt and flour. Fold the dry ingredients into the egg mixture, along with the vanilla extract.
Pour into the prepared cake tin and even out the mixture with a palette knife. Lay the plum halves skin side down on top of the cake. Bake for 30-35 minutes or until the cake is still slightly wobbly in the middle.
Meanwhile, make the cake topping. Melt the butter and once melted remove from the heat and mix with all the other ingredients until combined. `
Take the cake out of the oven and increase the heat to 200 C, Fan 180 C, Gas 6. Spread the topping over the cake and bake for 20-25 minutes or until golden and crisp.