Preheat the oven to 180C/350F. Place baking paper onto a full-size baking sheet pan.
Make the streusel: In a medium bowl, combine butter, flour, salt, vanilla essence, sugar, and cinnamon. Mix well until until crumbles form. Cover the bowl with a tea towel and store it in the fridge.
Prepare plums: Wash, pit, and quarter the plums. In a medium bowl, mix the plums with the icing sugar. Set aside.
Make the base: In a large bowl, combine butter and sugar. Add the eggs and vanilla and mix thoroughly. Slowly add the dry ingredients including flour, baking powder, and salt. Mix well until everything is well combined.
Assemble the cake: Pour the dough onto the baking sheet pan. Add plum quarters one by one and top it with the streusel.
Bake the cake for 50 minutes until the streusel is golden brown and slightly crunchy.
Serve: Once cooled, add a light dusting of icing sugar. Cut it into squares and enjoy it with a dollop of cream!
Notes
Chill streusel: Start by preparing the streusel to give them some time to rest in the fridge. Cooler streusel create a firmer and crunchier crumble layer once baked.
Lump-free batter: Make sure your base layer batter doesn't have any lumps to create a delicate and even cake layer.
Drain canned plums thoroughly to avoid the cake from becoming soggy.