In a large bowl, beat together the softened butter with the caster sugar and vanilla until smooth and creamy. This can be done by whisk or by hand, but be careful not to over-beat).

Separately mix together the dry ingredients (flours, xanthan gum, baking powder and salt). Tip: weigh into a large airtight container and shake vigorously.

Add the dry ingredients to the creamed butter bowl and 'massage' together with the back of a firm mixing spoon, flat knife or spatula until it all starts to come together as a dough. This will take some time, but be patient and do NOT add any additional liquid or other ingredients to the mix.

When the mixture is starting to come together, use hands to bring it all together into a smooth ball, but be careful not to 'knead' or overwork as the dough needs to remain cool and not to be over-compressed.

Line an 8 inch round loose-bottomed cake tin with a circle of baking paper.

Take the dough and press it into the base of the tin using hands and/or the back of a spoon, until it fills the tin as an even circle. (See NOTES if using the dough for individual cookies)

Smooth the surface with the back of a spoon, then place in the fridge for about 15 minutes to start to firm up a little. To 'scallop' the edges, the dough needs to have a little more stability, but not to be hard.

After 15 minutes, remove the tin from the fridge and carefully remove the round of biscuit dough from the tin, placing it on a flat surface where you can access the sides.

'Crimp' or 'scallop' the edges (I press a finger into each side to make opposite indentations and gradually work my way round the biscuit dough until the scallops meet).

Mark the top into segments with a sharp knife and prick the surface into a nice pattern for decoration.

Gently return the circle of dough to the baking tin and chill for a further 30 to 40 minutes.

Pre-heat the oven to 160 C (320 F) and bake the shortbread in the tin for about 55 minutes until the edges are just starting to darken slightly.

ALTERNATE - INDIVIDUAL COOKIES: Roll the dough (BEFORE chilling) to about ¾ to 1 cm thickness and use a cookie cutter to cut shapes. Carefully transfer the shapes to a lined baking tray with a slight gap between each. Chill the dough for about an hour before baking at 160 C for 30 to 35 minutes

Remove from the oven and sprinkle generously with additional caster or granulated sugar.

Cool the shortbread completely in the tin. (If there is any doubt about whether the shortbread is crisp enough, it can be left to cool in the turned off oven for an additional 20 to 30 minutes).