For the base:
150g plain flour
90g unsalted butter, cubed
50g icing sugar
For the topping:
175g caster sugar
Zest and juice of a lemon
Zest and juice of a lime
1tbsp plain flour
1/2tsp bicarbonate of soda
Pre-heat your oven to 180c and grease/line a deep baking tray or square oven proof dish.
Put the flour, butter and icing sugar for the base in the bowl of a food processor and blitz until it resembles fine breadcrumbs. Tip the mix into your tray/dish and press down firmly with the back of a spoon, making sure to push it right up to the edges and in the corners.
Bake for 15 minutes then remove from the oven, but leave the oven on.
While the base is cooking, make the topping. In a bowl, whisk the eggs until they turn frothy. Tip in the caster sugar a little at a time whilst still whisking, then add the lemon and lime zest and juice. Whisk this until well combined before adding the flour and bicarbonate of soda and giving one last mix.
When the base comes out of the oven, pour the topping over the base and spread into the corners with the back of a spoon. Return the dish to the oven and bake for a further 20-25 minutes until golden and just firm to the touch.
Leave to cool in the dish before turning out, dusting with a little icing sugar, trimming the edges and cutting into squares.