Preheat oven to 180°c fan and line a 22cm square shallow baking tin with greaseproof paper. If you don't have a square tin that's fine, you can use a round or oval tin or even baking dish like I did, you just won't get evenly shaped bars.
Put 225g of the flour into a mixing bowl with 85g of the sugar. Cut all the butter into small pieces and add to the flour and sugar.
Rub the butter into the flour and sugar using your fingertips until the mixture resembles fine breadcrumbs and there are no pieces of butter left. Be patient, this will take 7-10 minutes but makes all the difference to the base.
Tip the flour, sugar and butter mixture into the prepared baking tin/dish. Spread it out evenly and then press it down firmly all over. Then set aside.
Into a large bowl put the lemon zest and juice. Add the eggs and gently whisk until combined. Add the remaining 200g sugar and gently whisk again. Then add half a lemons worth of cold water, I just put the water in my lemon squeezer to measure.
Add the remaining 25g of flour to the lemon mixture and gently whisk one last time. If there are lumps of flour that's absolutely fine. Put this lemon topping aside for now.
Put the base prepared earlier into the oven for 15-20 minutes or until golden. Be sure not to over bake. Take it out of the oven and then straight away pour the lemon topping mixture over the base. Do not let the base cool before putting the topping on.
Put the tin/dish back into the oven for another 10-15 minutes until the top is just set and has a wobble. Remove from the oven and leave to cool before removing from the tin/dish. Once cool peel off the greaseproof paper and cut into bars.
