Make the streusel: combine the flour and sugar in a small bowl. Add the cold cubed butter and rub it in with your fingertips until the mixture resembles fine, sandy crumbs with a few small clumps. Refrigerate while you prepare the batter. 45 g butter, 63 g all-purpose flour, 63 g granulated sugar
Preheat the oven to 350°F / 180°C. Grease and line an 8-inch square cake pan with parchment paper.
Halve the plums and remove the stones. Slice into ¼-inch slices and set aside. 4-5 fresh plums
Combine the flour, baking powder, and salt in a small bowl and set aside. 250 g all-purpose flour, 2½ tsp baking powder, ½ tsp salt
Add the melted butter, oil, sugar, eggs, yogurt, milk, vanilla, and almond extract (if using) to a large bowl. Whisk well to combine. 200 g granulated sugar, 85 g butter, 28 g neutral vegetable oil, 2 large eggs, 1 tsp vanilla paste, 1/4 tsp almond extract, 120 g plain full-fat yogurt, 60 g milk
Fold the dry ingredients into the wet ingredients until just combined. A few small lumps are fine, don't over-mix.
Pour the batter into the prepared pan and spread evenly with an offset spatula.
Fan the plum slices across the top of the batter in overlapping rows.
Scatter the cold streusel lightly and evenly over the plums.
Bake for 45-55 minutes until golden and a skewer inserted into the centre of the cake between the plum slices comes out clean with a few moist crumbs. If the streusel is browning too quickly, tent loosely with foil and continue baking.
Let the cake cool in the pan for 10 minutes, then transfer to a wire rack. Dust with icing sugar before serving if you like.