Preheat the oven to 350F/180C and line a 9"x9" baking tray with parchment paper.

In a medium-size bowl, stir together the whole wheat flour and brown sugar.

Rub in the cold butter with your fingertips until you've got a texture like breadcrumbs.

Stir in 2 tablespoons of cold water and work the mixture with your hands until it forms a dough. If the mixture is too dry and crumbly, add more water 1 tablespoon at a time until you get a proper dough.

Roll out the dough until you've got a 9"x9" square. Place it in the lined baking dish and bake for 15 minutes, until the top just starts to turn golden.

Meanwhile, beat the white sugar, all-purpose flour, baking powder, eggs, lemon zest and lemon juice together for about 1 minute, until everything is combined.

Pour the lemon mixture on top of the baked crust and return to the oven for 20-25 minutes, until solid and just beginning to brown.

Leave to cool to room temperature for at least one hour in the baking tray before slicing into 9 squares and serving or storing inside a sealed container in the fridge.