Step 1

Grease and line base and sides of a 18cm x 28cm lamington pan. Beat butter and sugar in a bowl with electric beaters until pale. Sift in flour, cocoa and soda and beat slowly until just combined. Spread in pan and smooth with a spatula. Prick all over with a fork. Chill for 15 minutes.

Step 2

Preheat oven to 180°C. Cook shortbread for 25 minutes or until firm to touch. While it's still hot, use a knife to score it into 12 rectangles. Cool slightly, then remove from pan and cut into 12 pieces. Dust with icing sugar before serving. Shortbread will keep for 3-4 days in an airtight container.