1. Preheat the oven to 170C fan-forced or 190C conventional. Grease and line a 20cm by 20cm cake tin. Wrap a sheet of foil around the outside of the tin to catch any leaks while the plums cook.

2. Place all of the streusel ingredients in a bowl and rub together with your fingers until well combined. Spread the mix out on a lined baking tray and bake until cooked and slightly crunchy, about 15 minutes. Remove from the oven, cool and then crush with your hands until it becomes a rough crumb consistency.

3. Cover the base of the prepared tin with half of the brown sugar. Lay the plums, skin-side down in the tin, jamming them firmly in place in a decorative fashion – take some care, as this layer will form the top of your cake. Sprinkle over the remaining brown sugar and evenly distribute the 30 grams of butter in small knobs. Bake on a baking tray in the preheated oven for 20 minutes.

4. After 20 minutes, remove the tin from the oven, sprinkle half of the streusel mix onto the cooked plums and set aside.

5. For the cake, sift the flour, baking powder and salt into a bowl and set aside.

6. Place the butter, sugar, vanilla, lemon zest and liqueur in a food processor and process until pale and creamed. Add the eggs one at a time, followed by the sour cream and then the sifted flour mix. Don't be alarmed if it's a little stiff, it should be quite a thick batter.

7. Tip the batter into a bowl and fold through the remaining streusel crumble. Tip into the tin, smooth over the top and bake for 50–to 60 minutes, until an inserted skewer comes out cleanly.

8. Set aside to cool for 20 minutes and then turn out onto a cake plate, with the plums facing up. Serve warm with cream.

Tips

This cake is best on the day of cooking, and even better still warm from the oven.

Double the streusel mix to garnish the finished cake 

You can use a mix of different plums, just make sure that they're properly ripe.