Preheat oven 160 C. Grease and line a 30cm x 20cm rectangular slice pan and line base and sides with baking paper, extending 2 cm over the edge.
Cream butter and icing sugar for 2 minutes or until pale and creamy. Sift flour and baking powder over butter mixture and mix with your hands until combined.
Press mix into slice pan, and smooth over top with a pallet knife. Bake 25 minutes.
Remove from oven and using a small sharp knife, immediately slice in rectangular pieces, 5 cm crossways x 75 cm lengthways. Then run the knife gently 1/2 way into shortbread pieces, NOT TOUCHING BOTTOM, to make finger indent. Let biscuits cool in pan for 10- 15 minutes before moving to a cooling rack. Trim edges, if desired before serving.