I have adapted the base from a basic 'pound cake mix' that my mother and grandmother used, to a shortbread base - simply because I find it is better especially in the heat and humidity that we get in New Zealand in summer. The basic idea can be adapted to any fruit - apples with cinnamon, pears with ginger, peaches, apricots, all plum varieties and I have even done this cake with feijoas.
The easy shortbread version is based on the 'melted butter shortbread' on http://www.food.com/recipe/melt-n-mix-shortbread-418259. I tend to use only plain flour though.
Add to the shortbread mix a half teaspoon of ground cloves and line a 18cm x 30cm cake tin or deep baking tray with the shortbread.
Stone approximately 1 kilo of plums.
Cover the shortbread with a layer of plum jam before putting the plums onto the jam - open side down.
Sprinkle up to half a cup of sugar over the plums - depending on how tart you like the plums when cooked.
For the crumble mix by hand 300g flour, 200g softened butter, 100g castor (fine) sugar and 1/2 tsp ground cloves. (the cloves with the plum gives an amazing flavour) together till a crumble mixture is formed. Sprinkle crumble mix over the plums.
Bake at 150C for at least an hour or until the top crumble layer is golden. Leave to cool and sprinkle with icing (powdered) sugar before serving.
Serve with fresh whipped cream or ice cream.