Preheat oven: 180°C / 160°C (fan) / Gas Mark 4 / 350°F
In a large heatproof bowl, melt the butter for 30-50secs in the microwave and beat in the sugars until smooth and thickened.
Thoroughly beat in the egg and vanilla extract.
Sieve in the flour and cornflour, stirring until just combined.
Stir in the chopped Caramilk chocolate.
Pour the blondie mix into a greased and lined 8-inch square pan, smoothing the top gently.
Bake for 20-25mins until the top is lightly golden and there is only a slight wobble in the middle.
Allow blondies to cool on the side for 5mins before topping them with the whole pieces of Caramilk spaced out evenly (I do 4 by 4 for 16 pieces total) and allow to cool fully in the tin before removing.
If using, melt the remaining Caramilk for the drizzle slowly in a microwave (15secs at a time then stir) or using a double-boiler.
Drizzle the melted Caramilk over your cooled blondies. You can use a piping bag with a small round nozzle, a sandwich bag with the corner cut off or just a teaspoon.
Allow the chocolate to set for 5-10mins at room temperature, then cut and enjoy!