Pre heat the oven to 170 C (fan). Grease the 20 cm round cake tin and line the bottom of it with baking parchment. Cut 150 g of plums into a 1 cm dice, slice the rest into wedges and set both aside.
Using an electric mixer, cream together the butter and caster sugar until pale and fluffy (about 3 minutes). Beat the eggs, one at a time, until combined, then add the almond essence.
In a separate bowl, mix together the flour, almonds and a pinch of salt, then fold into the egg mixture until just combined. Spoon some of the cake batter into a bottom of the cake tin, just to cover it. Add a layer of diced plums on top, then pour the remaining batter on top. Arrange the plum wedges on top and bake for 50-60 minutes until risen and golden, or until inserted in the centre toothpick comes out clean.
Cool in the tin for about 10 minutes, then transfer to a cooling rack. When ready to serve, toast the flaked almonds in a pan, until golden. Remove from the pan. Add plum jam into the pan with 1 tbsp of water, heat it up and stir to combine it. Brush the cake with plum jam then sprinkle with toasted almonds. Enjoy!