Preheat oven to 320°F/160°C, combi steam setting. If your oven has variable steam settings, select 80%. If not, don’t worry! Just set to combi steam at the correct temperature and the oven will work out the humidity for you. Line the base of a 9x13” (22x32cm) pan with parchment paper, allowing the paper to extend up over the two long sides so you have some leverage to remove the blondies from the pan later.
Brown the butter: put butter in a small saucepan over medium heat. Let it melt then come to the boil and cook, swirling the pan frequently, until it's stopped foaming and the milk solids begin to brown. This will take about 5 minutes, and you'll know it's close when the foamy bubbles on top give way to smaller more distinct bubbles. It'll smell like caramel and nuts and the solids will drop to the bottom and change colour. At this point take it straight off the heat and pour into a heatproof bowl to avoid burning it.
Add the sugars to the butter and stir well until smooth. The mixture should be just lukewarm by now; if it’s not, allow to cool for a few more minutes before adding the eggs, vanilla and salt.
Stir through the flour and then the nuts. Try not to overmix, it should be just combined without streaks of flour.
Scrape the mixture into the prepared pan and smooth it out. Put the Nutella into a piping bag or a resealable bag with one corner snipped off. Squeeze it in thick ribbons over the batter. Use a skewer to marble the Nutella and batter together.
Bake the blondies until they’re just browned at the edges but look slightly under-baked, about 15 minutes. Err on the side of less baked than more, here – softer, fudgier blondies are great, overbaked hard ones are not.
Allow the blondies to cool in the pan before removing and cutting into bars or squares. If they last that long, they’ll keep for up to 4 days in the fridge, or can be frozen for up to 2 months.