To a medium size bowl add the plain flour, bicarbonate soda, cornflour and salt. Whisk them together and set aside. To another small bowl, whisk together the egg and vanilla extract and also set aside.
To a stand mixer bowl or a mixing bowl add the butter and both the sugars then cream them together using a paddle on a stand mixer or a hand-held mixer. Do this on a medium setting for around 3 minutes. To the creamed butter and sugar, add the egg with the vanilla extract and combine. Add the pecans and white chocolate and mix until they are all incorporated. Add the dry ingredients set aside earlier and mix until it has all come together to form a thick dough.
Line a tray with baking paper and roll the dough into small balls. I use a scale for this and weigh out 40g dough for each ball. Place each ball on the baking paper. At this point we won’t be baking them so you can place them close together. Place the tray with the dough balls in the fridge for 2 hours.
Preheat the oven to gas mark 4/350oF/180oC and line another baking tray with baking paper. If you only have one tray then remove the dough from the fridge and set the dough balls aside on a plate then line that baking tray with baking paper.
I bake around 6 cookies on one baking tray at once as they do spread a little so place 6 cookies on the baking tray ensuring they have plenty of space between. The rest of the cookie dough balls can be placed back in the fridge to bake once the first batch has baked. You can also store the excess dough balls in the freezer until needed Bake for 12 minutes then remove from the oven and allow to cool for 15 minutes on the baking tray then remove and allow to cool completely on a cooling rack.