1. Preheat the oven to 170°C. Lightly grease a 22cm-square cake tin and line with baking paper, letting the paper overhang slightly around the rim to assist with removing the streusel once cooked.
2. To make the topping, place 50g of the flour in a bowl, add the cold cubed butter and rub in until the mixture resembles coarse breadcrumbs. Mix in the brown sugar and set aside.
3. To make the cake, sift the remaining flour, the baking powder and a pinch of sea salt into a bowl. In a large bowl, cream the softened butter and caster sugar with electric beaters.
4. Add the eggs, one at a time, beating well after each addition. Tip in the dry ingredients, alternating with the sour cream, while being careful not to over-mix (some small lumps of flour are okay).
5. Pour the batter into the lined cake tin and scatter over the plums and 150g of the blackberries, then finish with the topping.
6. Bake for 55-60 minutes, or until a skewer inserted into the centre comes out clean.
7. Allow the streusel to cool a little before lifting it out of the tin. Serve warm or at room temperature with the remaining blackberries and cream.
