Preheat the oven to 180ºc/160ºc fan and line two/three large baking trays with parchment paper, leave to the side.
Whisk the plain flour, bicarbonate of soda, salt, and cornflour together in a bowl and leave to the side.
Melt your butter carefully, and then add to a bowl. Add in the sugars, and whisk the mixture together for two minutes. (I used my electric whisk, if whisking by hand, whisk for 3 minutes)
Add in the vanilla and egg and whisk again until combined.
Add in the flour mixture and beat until everything is evenly distributed and a cookie dough is formed.
Add in the white chocolate chips and the funfetti sprinkles and beat until distributed well!
Scoop the cookie dough using a cookie scoop for even portions, then roll into balls. Bake in the oven for 10-13 minutes.
Leave the cookies to cool on the trays for 5 minutes, then transfer to a rack to cool completely! Enjoy!
Notes
These cookies can last for 4-5+ days easily, but they rarely do in my house!