Preheat your oven to 180°C (gas mark 4, 350°F) and line a 9″ square baking tin with parchment paper.
Add butter to a large mixing bowl and heat in the microwave until melted. Leave the butter to cool down for 1 to 2 minutes, then add caster sugar and light brown sugar and mix until combined.
Add eggs and vanilla to the bowl and mix until combined and evenly distributed.
Fold plain flour into the mixture until combined.
Pour the blondie batter into a lined baking tray and spread evenly using a spatula or wooden spoon
Place spoonfuls of Nutella on top of the blondie mixture, using a skewer create a marble pattern. See the video below on how I did this.
Bake in the oven for 12 to 15 minutes for gooey and fudgy Nutella blondies or 17 to 20 minutes for cake-like fudgy blondies. Leave to cool down before cutting into squares.
Video
Notes
Storage & Freezing Instructions
If you have any blondies left to store, it's best to store it in an airtight container and keep for up to 5 days. To avoid sticking, place parchment paper between each layer.If you want to freeze these blondies, place it into a freezer-safe container and keep in the freezer for up to 3 months. When you are ready to eat it, take it out of the freezer and leave to defrost.
Nutella Swirls Tip
The bigger the dollops of Nutella, the bigger the pools of melted chocolate when baked. Don't marble and swirl out the Nutella too much if you want large areas of melted Nutella.