Step 1

Preheat oven to 180C/160C fan-forced. Grease a Swiss roll pan.

Step 2

Use electric beaters to beat the butter and lemon rind until pale and creamy. Gradually beat in the sugar until well combined. Add the egg and beat until combined. Beat in the flour, in 2 batches, until a soft dough forms.

Step 3

Roll the dough between two sheets of baking paper until large enough to line the prepared pan. Remove the top layer of baking paper. Place dough, with the paper on the base still on, into the prepared pan, pressing in well.

Step 4

Bake for 25 minutes or until light golden and the top is just firm. Set aside to cool.

Step 5

Whisk condensed milk and lemon juice together in a medium bowl. Pour mixture over shortbread base. Smooth top. Bake for 15 minutes or until the edges are slightly bubbling and the top layer is set. Set aside to cool to room temperature, then place in the fridge overnight to chill.

Step 6

Remove the slice from the fridge and use a hot knife to cut into small triangles to serve.