Wash, destone and halve or quarter the plums depending on size.
In a mixing bowl, whisk to combine the flour and baking powder. Set aside.
In a separate mixing bowl, combine the sugar and oil (and zest or vanilla extract if using), then add the sparkling water and gently combine.
Now, add the dry mix (flour and baking powder) to the wet mix, and gently whisk to combine into a smooth, lump-free batter.
Pour the batter into a lined oven tray. We use a large flat 12 x 15 inches (30 x 38 cm) sheet tray. For different tray or tin sizes, see the notes below.
Place the plum halves on top of batter. If using, sprinkle over the flaked almonds.
Cook for 25-30mins until cake comes out clean when tested, fruit is nicely cooked and the cake just starts to brown a little bit.
With the cake fresh out of the oven and still hot, generously sprinkle on icing sugar, and some cinnamon if using. The icing sugar melts on the juicy hot fruit.
The cake can be served warm immediately, or transferred to a wire rack to cool.
Tray / tin sizesThis type of cake is typically baked on a large sheet tray that spans the entire width and depth of the oven. 12 x 15 inches (30 x 38cm) is a typical tray size.You can use two 9-inch square cake tins with similar, good results.This cake is intended to be only a few centimeters (or less than an inch) thick. If you prefer a thicker cake, simply use a smaller tin size and bake for slightly longer until a skewer comes out clean.Other tipsCheck out the post above for substitution ideas and tips on plums.This cake freezes well. When cool, cut into slices and freeze in a container, with layers of baking paper between them.Try other soft fruits as topping. Blackberries with some lemon zest in the batter is a delicious variant.