Preheat the oven to 180ºC and line a 23 cm (9-inch) square baking pan with baking paper.
In a medium bowl, whisk the sugar and eggs until well combined.
Melt the butter in a small saucepan or microwave, then let it cool slightly. Pour into the egg mixture and whisk until smooth.
Add the flour and mix until just combined. The batter will be very thick—this gives the blondies their soft, fudgy texture.
Spoon the batter into the prepared pan and smooth the top. Add spoonfuls of Nutella and use a knife or skewer to swirl it through the batter. Don’t overmix—keep visible swirls for the best texture and flavour.
Bake for 25–30 minutes, until the edges are set and the centre is still slightly soft. A knife inserted into the centre should come out with a few moist crumbs.
Cool completely in the pan before slicing into 9 squares.