Pre-heat your oven to 180°c (160°c for fan assisted oven, Gas Mark 4 or 350°F) and line a square 7-inch cake tin with greaseproof paper. Set aside.
Half your plums, discarding the stone and leaving the skins on. Set aside.
In a large bowl, cream together the butter/margarine and sugar, until light and fluffy. Add your eggs, one at a time, mixing until combined before adding the next. Add the vanilla and mix through.
Sift the in the flour, before gently folding through to create a smooth batter. Transfer the batter to your pre-lined cake tin, smoothing the top.
Place your cut plums skin-down on top of the cake batter, pressing them down a little. I like to do 3 rows of 3 (nine plum halves in total) but you can lay them out however you like. Sprinkle the top of the cake with an additional tablespoon of caster sugar.
Bake in your pre-heated oven for 35-45 minutes, until golden brown and the plums look nice & jammy. Allow to cool in to the tin a little, before serving. Best enjoyed still slightly warm, with a scoop of ice cream.