Preheat the oven to 180C/350F/Gas 4, and line a baking tray with parchment paper or a silicone mat. Set aside.
In a medium bowl, whisk together the flour, baking powder, baking soda, cornflour, and salt. Set aside.
In a separate large bowl, whisk together the butter and sugar. Add the egg, egg yolk and vanilla, and whisk until combined.
Add the dry ingredients and mix until just combined. Fold in the sprinkles.
Roll the dough into 1.25-oz/37g balls (or use a medium cookie scoop), and place onto the prepared baking tray. Bake for 10 minutes.
Allow to cool on the baking tray for 5 - 10 minutes before transferring them to a wire rack to cool completely.
Notes
Storage: Cookies stay fresh kept in an airtight container at room temperature for up to 1 week.Make ahead: Cookie dough balls can be kept in the fridge for up to 2 - 3 days, or frozen for up to 3 months. Bake straight from frozen.