Blueberry streusel cake

1. Preheat oven to 180C. Grease a deep 23cm springform tin.
2. To make the streusel, mix the ingredients together in a food processor until clumps form (you may need to add a teaspoon of water). Tip into a bowl and put in the fridge to keep cool until needed.
3. Mix the flour and baking powder together. Cream the butter and sugar in a bowl. Add the eggs one at a time with a spoonful of the flour with each addition and mixing well inbetween.
4. Add the rest of the flour, sour cream, and lemon zest and beat well.
5. Fold 2/3 of the blueberries into the mixture and pour into the greased tin. Dot the remaining blueberries over the top and sprinkle over half of the streusel mix. If the clumps are too large, break them up with your fingers.
6. Bake for 50 - 60 minutes. After 40 minutes, quickly remove the cake from the oven and sprinkle over the remainder of the streusel. The cake is ready when a skewer poked into the cake comes out clean.

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