Preheat your oven to 180C/350F/Gas mark 4 and lighlty greese two 8 inch (20cm) cake tins.
In a large mixing bowl add in the boxed cake mix.
Add the eggs, oil, milk, lemon extract and lemon zest. With a gentle hand mix everything together until everything is well combined and you have a smooth batter.
Gently fold in the blueberries and devide the batter equally between the two cake tins. Place the tins into the center of your oven and bake for 25 minutes.
Check your cake with a pick, if it comes out clean then your cakes are baked through. If not give it another minute or two.
Release the cakes from their tins and allow them to fully cool down on a wire rack.
Once cold, spread the lemon curd on one half of the cake and sandwich it together with the other half.
In a small bowl mix the icing sugar, water and lemon juice together and drizzle it all over the top of the cake.
If you want to then add a few blueberries to the top and a little lemon zest then do so, its completly optional.