Preheat the oven to 180°C/350°F/Gas 4 and lightly grease and line a 20cm square cake tin.
To make the crumble:
Place the flour in a large bowl and using your fingertips, rub the butter in until it resembles fine breadcrumbs. Mix in the sugar, lemon and cinnamon (if using) and leave aside.
To make the cake:
Sieve the flour and bread soda into a bowl, stir in the blueberries.
Melt the butter in the microwave or place it in a saucepan over a low heat and melt. Remove from the heat and mix in the sugar, eggs, yogurt, lemon juice and vanilla extract.
Make a well in the centre of the flour bowl and pout in the butter mixture, mixing well with a spoon until well mixed but being careful not to burst the blueberries if being used.
Spoon half of the cake mixture into the prepared tin, leveling out the top using a spatula or back of the spoon. Sprinkle over half the crumble topping. Spoon the remaining mixture on top - you will need to drop it in spoonfuls to avoid mixing the crumble topping underneath. Spread as evenly as you can. Sprinkle on the remaining crumble on top of the cake.
Bake for about 55-60 minutes or until firm to the touch and a skewer should come out clean.
Leave in tin to cool for around an hour and finish cooling on a wire tray.