Preheat your oven to 180ºC. Line a 23 cm (9-inch) square baking tin with baking paper.
In a medium bowl, mix the sugar and eggs until well combined.
Melt the butter in a small saucepan or microwave, then let it cool for a few minutes. Once cooled, pour the butter into the sugar and egg mixture. Whisk well.
Add the flour to the bowl and mix until thoroughly combined. The batter will be very thick.
Pour the batter into the prepared baking tin and smooth the surface. Drop large spoonfuls of Nutella on top of the batter and use a toothpick or knife to swirl it through the mixture.
Bake in the preheated oven for 25-30 minutes, or until the blondies are set with a slight crackle on top. A knife inserted into the center should come out clean.
Let the blondies cool completely in the pan before cutting them into 9 equal squares.