Heat oven to 190 ºC

If using, toast the pecans in a pan. Leave to cool, then chop fairly coarsely.

In a large bowl, mix the flour and sugar together.

Add the cold, cubed butter and use your fingertips to rub the butter into the dry ingredients. Lift the mixture as you do this to keep it aerated. Stop as soon as it resembles fine, sandy breadcrumbs - do not overwork it.

If using, scatter the chopped pecans over the top. Use a spatula or your hands to gently fold and toss the nuts through the crumbs until they are evenly distributed.

Squeeze the mixture together with your hands until it just starts to clump into a cohesive, shaggy dough. Do not knead it; just press it together until it holds its shape.

Transfer the dough to your prepared tin. Use the back of a spoon or your fingertips to press it firmly and evenly into the corners and across the bottom of the tin.

Use a bench scraper or knife to cut the dough into fingers. Tip: make sure your fingers are wide enough that a fork can prick them. Next, prick the shortbread with a fork, about half way through (this is called "docking"—it helps it bake evenly).

Pop the tin into the fridge for at least 20 minutes. This allows the butter to firm up again, which helps the shortbread keep its shape and maintain that signature texture.

Bake for 30–40 minutes until a very pale, light golden color.

Remove from the oven, sprinkle the top with the extra caster sugar while still warm, and leave to cool in the tin for about 10 minutes. Cut through your score lines while still slightly warm, then leave to cool completely before removing from the tin.