To a bowl, add and whisk together the plain flour, bicarbonate soda, salt and cornflour then set aside. In another bowl, cream together the butter and sugar until creamy and fluffy. Add in the egg then the vanilla and continue to mix until well combined.
Pour in the dry ingredients and continue to mix until all the ingredients are well combined then add and mix in the sprinkles.
Roll the dough into balls weighing around 40g each then roll in more sprinkle if you wish and place the dough balls on a large plate lined with baking paper. Place the cookie dough in the fridge for 1 hour.
While the cookie dough is in the refrigerator, line a baking tray with baking paper and preheat the oven to gas mark 4/350oF/180oC/160oC fan.
Place the cookies that you want to bake on the baking tray ensuring there is space for them to spread a little. If you do not have enough space on the tray then you can place the remaining cookie dough balls back in the fridge and bake them in batches or you can freeze the rest and bake as needed.
Bake in the lower part of the oven for 12 minutes. Once you take them out the oven, allow them to cool on the baking tray for 10 minutes then use a spatula and transfer the cookies to a cooling rack to completely cool.