Preheat the oven to 180C fan/200C/400F/gas mark 6 and line an 8 inch baking tin.
To make the blondie layer: Start by whisking the eggs on high speed with an electric mixer for 5 minutes, until pale and fluffy. Add the sugar and whisk again.
Add in the melted white chocolate, melted butter and lemon extract if using, and whisk until combined. Fold in the flour before folding in the lemon zest and chopped chocolate. Pour into the baking tin and set aside.
To make the lemon tart topping: First whisk the flour and sugar together. Then add the eggs and whisk again until combined. Finally, whisk in the lemon juice. The final result will be pretty runny. Pour this over the blondie base.
Bake for 35 minutes, until the edges are browning. The middle may wobble just slightly but shouldn’t be overly wobbly. Leave to cool entirely before dusting with icing sugar and slicing (use a very sharp knife for clean slices).