Preheat your oven to 190C/170C fan, and line a 9x9inch square tin with parchment paper, and leave to the side for now.
Whisk together the Plain Flour, Bicarbonate of Soda, Salt and Cornflour together in a bowl and leave to the side for now.
Melt the Unsalted Butter till liquid, and add to a bowl. Add in the Light Brown Sugar and Granulated Sugar and whisk the mixture together for two minutes by an electric whisk, 3 minutes by hand.
Add in the Egg and Vanilla and whisk again till they're combined in.
Add in the mixed together Dry Ingredients and mix until a really thick cookie dough is formed.
Add in the chopped Kinder Chocolate, and chopped Kinder Bueno, and fold through. Reserve some of the Kinder Chocolate for the top of the Cookie Bars.
Push the mixture into the 9x9inch Tin, and add on the last few bits of Kinder Goodness.
Bake the Cookie Bars in the oven for 17-22 minutes. I usually do 19 minutes so they stay reasonably gooey.
Once baked, leave in the tin for 10 minutes to cool a bit, and then remove and cool fully on the wire rack. Enjoy!