Preheat the oven to 200C/180C Fan/Gas 6. Line an 8′ Square Baking Pan (9″ will work for thinner bars) with baking parchment.
In a large bowl, or the bowl of a stand mixer, combine the butter, brown sugar and granulated sugar (medium speed) for about 3-5 minutes until fully combined.
Add the egg and vanilla and mix again.
Fold in the flour, baking soda and cornflour and fold together. Then gently fold in the milk chocolate.
Press half of the dough into the lined tin. This may require your hands as it is pretty firm. Layer over the Kinder chocolate bars - keep them intact and line up side by side - leaving a little space at the edges. Cover with the remaining cookie dough.
Bake for about 25 minutes, until entirely baked. Leave to cool on a wire rack for at least 45 minutes before attempting to cut (they retain their heat and will fall apart if cut any earlier).
