Cube the 85g of butter and put into a pan with the 75ml of milk. Put the hob on the lowest heat and very gently melt. Don't let the butter get too hot and burn.
Using an electric mixer, whisk the eggs, 175g caster sugar and vanilla extract in a bowl until it becomes thick and like a mousse. This will take at least 5 mins, but this is a really important part of the recipe to ensure it rises beautifully in the oven. So as much as your arm aches, please ensure it's thick and mousse- like.
While continuing to whisk, gently pour the milk and butter bit by bit into the eggs and sugar. Once combined stop whisking, and sift the flour, cinnamon and baking powder over the mixture.
Very gently fold the mixture to combine the flour until there are no lumps.
Pour the mixture into the prepared baking tin. This will look like hardly any mixture but I promise it will rise!
Cut the plums into nice thin wedges (think like a Terrys chocolate orange kind size!), discarding the stones.then arrange them over the batter, gently (and I mean gently) push a corner of the wedge just into the batter. You will notice them sink slowly, and during the bake they will go to the bottom.
Arrange the plum slices over the batter, gently (and I mean gently) push a corner of the wedge just into the batter. You will notice them sink slowly, and during the bake they will go to the bottom.
Sprinkle the tbsp of sugar we put to one side roughly over the top, if it ends up in clumps this will be delicious so don't stress on being perfect.