Preheat the oven to 325 degrees.
Line a 7x11 pan with parchment paper.
In a pan, melt all the butter over medium heat, taking care that it doesn’t burn. Set aside to cool slightly.
In a mixing bowl, stir together the eggs, demerara sugar, and granulated sugar.
Stir in the melted butter, salt, vanilla, flour, baking soda, and baking powder. (Mixture will be quite thick.).
Spoon the batter into the prepared pan in an even layer.
Dollop the Nutella over the surface of the batter, and swirl the tip of a knife in the Nutella to get a pretty, freestyle pattern.
Bake for 25-30 minutes, checking frequently to make sure the blondies don’t overbake.
The blondies are done when the batter is puffed and golden, and is still a little moist in the middle.
The blondies will collapse slightly once they come out of the oven, and will harden into slice-able pieces in about an hour.