Preheat the oven to 350F (180C). Place baking paper onto a half-sheet pan (13x18 inches or about 33x46cm) or grease it with extra butter.
To make the streusel topping, combine softened butter, sugar, flour, vanilla extract, cinnamon, and salt in a medium-sized bowl by hand. Rub the ingredients between the fingers until 1/4 inch crumbles form. Cover the bowl with a tea towel and store it in the fridge.
1/2 cup unsalted butter, 1/2 cup granulated sugar, 1/2 cup all-purpose flour, 1 tsp vanilla extract, 1/2 tsp ground cinnamon, 1 pinch salt
To prepare the plums, wash, pit, and quarter them. In a medium bowl, mix the plums with the icing sugar. Set aside. If you're using canned plums, drain them thoroughly, check for any leftover stones, and quarter them. You can skip the powdered sugar.
1 kg plums, 1 tbsp powdered sugar
To make the base, cream the softened butter and sugar in a large bowl using stand mixer or handheld mixer. Add the eggs one at a time, mixing in between each additions, and stir in the vanilla at the end. Add the dry ingredients gradually, including the flour, baking powder, and salt. Mix until just combined.
To assemble the cake, pour the dough onto the baking sheet pan and smooth the top with a spatula. Add the plums one by one and top the cake with the streusel.
Bakethe cake for 45-50 minutes until the streusel is golden brown and slightly crunchy and a toothpick comes out clean.
Cool for 15 minutes and add a light dusting of icing sugar. Cut it into squares and enjoy it warm with a dollop of whipped 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.Plums: Drain canned plums thoroughly to avoid the cake from becoming soggy.Storage: This moist cake stores well in the fridge in an airtight container for 2-3 days or up to 3 months in the freezer.