Pre-heat oven to 180°C. Grease 2, 19cm (W) x 19cm (L) x 4cm (D) baking trays. Line each with baking paper.
Using hand-held electric beaters, cream butter and sugar together 5 min until pale.
Add FATE mix and custard powder. Using a large metal spoon, gently stir until mixture comes together and a ball of dough is formed.
Dust a clean work bench with extra FATE mix. Place dough on top of FATE mix and shape gently 1-2 min. Cut ball in half. Shape each piece to form 2 smaller balls.
Place one piece of dough into one of the prepared tins. Using the palm of your hand, flatten the dough until it has spread evenly across the base of the tin.
Repeat with remaining dough (and tin).
Bake 15-20 min until just golden on top. Remove from oven. Slice each large square into 16 even sized smaller squares. Leave in tin to cool, 10 min.
Remove from tin and place each shortbread biscuit on a wire rack to cool completely. Sprinkle with extra sugar if desired.
Shortbread biscuits store well in an airtight container for 2-3 days.