Spread the pecan nuts on a baking tray and place in the oven to roast for 6-8 minutes, shaking the pan halfway through, until the pecans smell nutty and are slightly darker. Remove and allow to cool completely.
In the bowl of a food processor, pulse together the butter and sugar until just combined.
Add the flour, cornstarch and vanilla then mix until just combined with dry pockets here and there.
Add the pecans and pulse until finely chopped but not completely fine.
Roll the dough into cookie balls, just less than 1 tablespoon worth of dough per cookie.
Place the dough balls on a lined cookie sheet and refrigerate for at least 30 minutes.
Bake in a preheated oven (180°C/350°F) for 12-15 until the bottoms are just barely golden.
Remove from the oven and allow to cool for 5 minutes until just cool enough to handle then roll in the powdered sugar. Place on a wire rack to cool completley.
Once cooled completely, roll in the powdered sugar a second time for a thick coating.