For the Streusel

1. Add the dry ingredients, flour, brown sugar, cinnamon, and salt in a bowl with cold butter.
2. Rub the dry ingredients in cold butter using your finger tips until the mixture becomes crumbly.
3. Chill in the fridge while preparing the batter.

For the Cake

1. Preparation: Preheat oven to 170°C fan.  Line a 1 lb loaf tin with parchment paper, leaving an overhang for easy removal.
2. Rub lemon zest into the sugar for 1–2 minutes to release the oils.
3. Lightly mist blueberries with water and toss with 1 tbsp flour to prevent sinking.
4. Make the Batter: Beat the butter and the lemon-infused sugar until pale and light.  Scape down the sides and bottom of the bowl before adding the rest of the ingredients. 
5. Add the eggs, one at a time, mixing well after each addition. 
6. Add sour cream and vanilla extract, mix until well incorporated.
7. Sift in flour, baking powder, and salt. Beat until just combined. Gently fold in flour-coated blueberries with a spatula until evenly distributed. 
8. Pour the batter into the lined baking pan.  
9. Sprinkle the streusel evenly over the top.  
10. Bake and Cool the Cake: Bake in a preheated oven at 170°C fan for about 45–50 minutes, or until a skewer inserted into the centre comes out clean.
11. Let the cake cool slightly in the pan for a few minutes, then lift out using parchment overhang. Remove the parchment paper underneath and allow the cake to cool completely on a wire rack before slicing.*