Start by toasting the pecans (or nut of your choice), if using. Heat oven to 150°C / 130°C Fan / 300°F / Gas Mark 2. Spread the pecans out on a baking tray and toast in the oven for 8-10 minutes. Allow the nuts to cool, before chopping finely.
To make the cookie dough: Cream the butter for a minute until softened. Now add the 90g portion of icing sugar and cream for another 2-3 minutes, until fluffy and lightened in colour.
Now add the vanilla extract and mix to combine. Add the flour and continue to mix until it's incorporated fully. Finally, fold through the chopped nuts, if using.
Cover and chill the dough for 30 minutes, or up to 3 days. If you chill the dough longer than a few hours, it will need about 30 minutes at room temperature to warm up before rolling. Preheat oven to 180°C / 160°C / 350°F / Gas Mark 4. Line 3 large baking trays with parchment paper.
Take 20g portions of the dough and roll into balls. Place the balls of cookie dough at least 2 inches apart - you'll need about 3 large baking trays to bake all the cookies at once.
Bake the cookies for 15 minutes, until the bottoms are just turning golden, and the tops are lightly browned. Allow the cookies to cool on the baking trays for 5 minutes, then take the remaining icing sugar (150g), and roll each cookie lightly and very carefully in the icing sugar. They will be slightly sticky at this point.
Now place each cookie on a wire rack and allow to cool completely. Once cool, take the cookies and coat in the icing sugar once again - this is when the sugar really coats the cookies.
Enjoy the cookies right away, or keep them stored in an airtight container/or covered plate for up to one week.