In a large mixing bowl, whisk together the flour, bicarbonate of soda, cornflour and the salt - only add a pinch of salt if using salted butter.
In a medium mixing bowl, whisk the melted butter with both types of sugar until the sugar has dissolved. Add the whole egg, egg yolk and the vanilla extract, and whisk again to combine.
Now add the wet ingredients to your bowl of dry ingredients. Gently fold together to combine. Add your white chocolate chips and chopped Caramilk, stir to mix through. Cover the bowl and chill the cookie dough for at least 2-3 hours, or up to 3 days. I leave mine overnight and then allow it to sit at room temperature for 15 minutes before rolling.
When ready to roll the cookie dough, take 60g portions and roll into balls. Preheat oven 160°C / 150°C Fan / 325 °F / Gas Mark 4.
Space the cookie dough out on 2-3 large baking trays - you might need to bake the cookies in batches. Bake for 12-14 minutes or until lightly golden around the edges - the cookies will appear very soft, but that's what you want. Press extra white chocolate chips and chopped Caramilk on top of each cookie while they're still hot. Leave the cookies to cool for 10 minutes on the baking trays, then they're ready to serve.
The cookies will keep for up to 3 days, stored in an airtight container or a covered plate at room temperature. Freeze the cookies in an airtight container/or freezer-safe bag for up to 3 months - thaw the cookie before baking.