Line an 8x8-inch square baking pan with parchment paper. Spread the Nutella onto the paper to create an even layer.
Place in the freezer for at least 1 hour before making the blondies.
Preheat the oven to 180C/350F/Gas 4. Line an 8x8-inch square baking pan with parchment paper. Set aside.
NOTE: If you only have one 8x8-inch pan, remove the Nutella from the pan (still attached to the paper) and place the Nutella back in the freezer. Line the pan again with parchment paper.
In a large bowl, whisk together the butter and sugars until combined. Add the egg, egg yolk, and vanilla, and whisk until smooth and combined.
Add the flour and salt, and fold in until completely combined.
Spoon half of the batter into the prepared pan and spread out into an even layer.
Remove the Nutella from the freezer, peel off the paper, and place it on top of the batter.
Top with the remaining batter and spread out into an even layer.
Bake for 35 - 40 minutes, or until the edges are browned and the centre is no longer jiggly - just set.
Allow to cool completely in the pan on a wire rack before cutting into bars.
TIP: To speed up the cooling time (and for cleaner, neater slices), place the pan in the freezer for 1 hour.
Notes
Blondies can be stored in an airtight container at room temperature or in the fridge for up to 1 week. They also freeze well for up to 3 months. Thaw overnight in the fridge before serving.