Grease and line a lamington tin (18 cm x 28 cm) with baking paper.
Sift the flour, cocoa, soda and salt into a bowl and set aside.
Beat the softened butter and sugar in an electric mixer until creamy in texture.
Add the flour mixture gradually and beat slowly until just combined.
Press into the prepared lamington tin, smooth out the top and refrigerate for 15 minutes. At this time, preheat the oven to 180℃ (160 ℃ fan-forced).
Remove from the fridge, use a knife to score the chilled dough into 12 pieces. This makes it much easier to break apart once it's cooked.
Prick the dough all over with a fork and sprinkle with the wattleseed. Cook the shortbread for 25 minutes until it's firm to touch.
Allow the shortbread to cool slightly before turning it onto a wire rack. Cool completely before storing it in an airtight container.
The shortbread will keep in an airtight container for up to a week.