Plum and Blackberry Upside-Down Cake

Name: Plum and Blackberry Upside-Down Cake
Prep time: 15 min
Total time: 15 min
Yield: 4 porties
Cuisine: Belgian
Ingredients: unsalted butter; light brown muscovado sugar; plums; fresh blackberries; caster sugar; medium egg; vanilla extract; plain flour; milk; baking powder