For the cake:
225 g butter, softened
225 g caster sugar
275 g self-raising flour
2 tsp baking powder
4 tbsp milk
2 lemons, grated zest only
For the crunchy topping:
175 g granulated sugar
2 lemons, juice only
Cut a rectangle of non-stick baking parchment to fit the base and sides of a 30 x 23 x 4 cm roasting tin. Grease the tin and line with the paper, pushing it neatly into the corners of the tin. Preheat the oven to 180C/160C fan/gas 4.
Measure all the ingredients for the tray bake into a large bowl and beat well for about 2 minutes, until well blended.
Turn the mixture into the prepared tin, scraping the sides of the bowl with a plastic spatula to remove all of the mixture. Level the top gently with the back of the spatula.
Bake in the middle of the oven for about 35-40 minutes, or until the cake springs back when lightly pressed and is beginning to shrink away from the sides of the tin.
Leave the cake to cool in the tin for a few minutes then lift it out of the tin while still in its lining paper. Carefully remove the paper and put the tray bake onto a wire rack placed over a tray.
To make the crunchy topping, mix the lemon juice and granulated sugar in a small bowl to give a runny consistency. Spoon this mixture evenly over the tray bake whilst it is still just warm. Cut into squares when cold and store in an airtight tin.