Preheat the oven to 180 degrees C. Grease and line a 23cm cake tin with baking paper.
Topping: In a saucepan over a medium heat, combine the butter, sugar and golden syrup and stir until melted and a syrup has formed.
Arrange the plum wedges in the base of the prepared cake tin in a pleasing pattern. Pour the syrup over, taking care not to move the plums.
Cake: In a large mixing bowl, sift together the flour and baking powder then add the salt and mix well.
In a separate bowl, cream together the butter and sugar until pale and fluffy. Add the eggs, one at a time, mixing well after each addition then mix in the vanilla extract.
Add the dry ingredients to the wet ingredients, alternating with adding the milk. Mix until smooth. Carefully and slowly, pour the cake mixture over the plums taking care not to move the plums. Smooth the top.
Bake in the preheated oven for 1 hour, or until golden brown and a skewer inserted into the centre comes out clean. Remove from the oven and allow to rest for 10 minutes in the tin.
To Serve: To flip the cake, place a plate over the top of the cake tin, ensuring that any overhanging baking paper is out of the way. Holding the plate firmly to the cake tin, flip the cake over and allow it to gently drop onto the plate. Remove the tin then peel back the baking paper and discard. Slice and serve.
It is not advisable to use a springform or loose-bottomed cake tin for upside down cakes as they can be prone to leaking.
To freeze, wrap the cake up tightly in tin foil and place in the freezer. When ready to eat, allow about 8 hours to defrost to room temperature.