Preheat the oven to 180C/160Fan/350F.
When the butter has cooled, add the sugar to the melted butter and beat well. Add the egg and vanilla extract and beat well until the mixture is shiny. Add the flour and salt and mix until well combined. Stir in the chocolate chips.
Place half of the blondie mixture in the tin and smooth it out as best as you can (I find it easier to use my hands here)
Take your frozen Nutella and place them on top of the blondie. Imagine the blondies once they are baked and sliced – you ideally want them all to have a generous portion of Nutella inside them!.
Top with the remaining blondie batter, trying to smooth out the top as well as you can and ensuring the Nutella in the middle is covered. Again, I found it easiest to place spoonfuls of the batter over the top and spread it out with my hands.
Finally, top the blondie withthe 25g Nutella, dropping it over the top in little teaspoons.
Take a skewer or tip of a knife and swirl gently, trying not to disturb the Nutella in the middle. Finish with a sprinkling of chocolate chips.
Place in the oven and bake for 20-25 mins or until the edges look set and the middle is slightly firm.
Remove from oven and cool completely before cutting into squares.