Cut a rectangle of non-stick baking parchment to fit the base and sides of a traybake tin.
Grease the tin and then line with the paper, pushing it neatly into the corners of the tin. Pre-heat the oven to 160°C/325°F/Gas 3.
Measure all the ingredients for the traybake into a large bowl and beat well for about 2 minutes until well incorporated
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 pre-heated oven for about 35-40 minutes or until the traybake springs back when pressed lightly with a finger in the centre and is beginning to shrink away from the sides of the tin.
Allow the traybake to cool in the tin for a few minutes then lift the traybake out of the tin still in the lining paper.
Carefully remove the paper and put the traybake onto a wire rack placed over a tray (to catch drips of the topping).
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 traybake whilst it is still just warm. Cut into about squares when cold.