Start by bringing your butter to room temperature so it mixes easily.
In your bowl, beat the softened butter and icing sugar together until the mixture is smooth and light. You can use a food processor on medium speed at this stage or use a hand whisk.
Then add the vanilla extract and mix briefly.
Sift the flour, cornstarch and salt into the bowl. Add the ground nuts, then mix gently until the dough comes together. You want it smooth, cohesive, but not overly kneaded. Too much kneading can result in a tougher texture.
Divide the dough into three equal pieces, and roll them into ropes.
Lightly press with a fork to make little fork marks on each rope, then cut each rope into 10 equal pieces.
Place them on a parchment-lined tray with some space between.
If your dough is very soft or warm, place the tray in the fridge for 15-20 minutes to help prevent excessive spreading.
Preheat your oven to about 160 °C (320 °F) for a fan-oven or 175 °C (350 °F) for a conventional oven.
Bake for approximately 15-20 minutes, until the edges are just set but the centre still looks pale.
The cookies should stay light-coloured; they don’t need to get golden-brown. Over-baking will dry them out and change their delicate crumb.
As soon as the cookies come out of the oven, dust them with powdered sugar (this helps the sugar stick while the heat is still present). Then place them on a cooling rack and allow them to cool completely.
Once fully cool, you can dust again for extra effect if you like. Cooling fully is important because, as they cool, they settle into their signature crumbly texture.