Preheat Oven to 350F. For the topping, combine the ingredients until they resemble bread crumbs
Now take the plums, halve it and take out the pit.
Cream the butter and sugar until fluffy
You can add the eggs one by one (if using) or else add the plum sauce and vegetable oil along with vanilla and beat it well
Slowly in 2-3 batches, sift, then fold in the flour and baking powder together to this mixture.
Pour the mixture in a Cake pan (10X2 preferable- greased and topped with a wax paper. Grease the wax paper too.) and arrange the plums on top.
Bake in the oven for approx 45 min until a skewer inserted in the center comes out clean.