Pre-heat your oven to 190 degrees Celsius/Gas Mark 5/375 F. Grease and line a rectangular baking tray with parchment.
Chop the fresh plums in half and remove the stones.
Put the caster sugar, eggs, oil and vanilla extract into a mixing bowl and whisk together well for 3-4 minutes until a pale and fluffy. Add in the zest and mix again.
Sift in the flour and baking powder and gently fold or mix with a metal spoon until all the flour is incorporated. Sprinkle in the poppy seeds and mix again to combine.
Pour the batter carefully into the tray and gently press the halved plums into the batter.
Bake in a pre-heated oven for 35-40 minutes, or until a toothpick comes out clean when inserted into the centre of the tray bake.
Leave to cool. Dust the cake with icing sugar mixed with a teaspoon of cinnamon and serve with cream and/or a warm plum compote.