1. Place all the ingredients into a freestanding food mixer and beat until the mixture just holds together. Alternatively beat the butter and sugar by hand then mix in the remaining ingredients. Turn onto a lightly floured work top and roll out to a 23cm (9 inch) circle.
2. Place a sheet of Bake-o-Glide onto the AGA Cold Plain Shelf or baking tray. Lift the shortbread circle onto the Bake-o-Glide and crimp the edges.  Mark into sections and prick with a fork.
3. Slide the cold plain shelf onto the lowest set of runners in the baking oven and bake for about 20-25 minutes, turning the tray once after 10 minutes, until a pale golden colour.AGA R5 Series 2-oven:  Place the baking tray onto the oven grid shelf set on the floor of the roasting oven. Slide the cold plain shelf on the second runners down, so it sits above the shortbread. Bake for 15-20 minutes turning the tray once after 10 minutes, until pale golden at the edges.
4. Take the shortbread from the oven, sprinkle immediately with caster sugar.  Mark into 8 or 12 triangles, do not cut down onto the Bake-o-Glide, as it will mark. Allow to cool then lift and complete the cooling on a wire cake rack.

This freezes well.

• This quantity will also make 24 Christmas tree shapes, using a cutter 8cm long.