Line an 8x8 inch (20x20 cm) square baking pan with parchment paper, leaving an overhang around the edges.
Make the Base
In a medium bowl, beat together the butter and sugar until fluffy. Mix in the vanilla extract.
With the mixer on low speed, mix in the flour, cornstarch and salt. The mixture may be a bit crumbly, which is ok.
Press the mixture into the bottom of the prepared pan.
Bake in the oven for 20-25 minutes or until the top looks set.
Remove from the oven, leaving the oven turned on.
Crumb Topping
In a medium bowl, stir together the flour, brown sugar, white sugar and cinnamon.
Stir in the melted butter - the mixture should be like wet/damp sand.
Assemble & Bake
Spread the Nutella over the baked shortbread base. Be careful, the pan will be hot.
Crumble the crumb topping mixture over top of the Nutella, squeezing it in your fist to form crumbles.
Bake in the preheated oven for 15 minutes, or until the top is golden.
Remove from the oven and cool the bars fully in the pan. They should be completely cooled.
Optionally, cover the pan and place in the fridge for at least 1 hour (after the bars have fully cooled) before slicing for cleaner cuts.
When ready to slice, use the parchment overhand to lift the bars out of the pan and place on a cutting board. Slice bars using a sharp, thin knife (not a butter/table knife).
Notes
I do not recommend using homemade or natural chocolate hazelnut spread. It can separate and make the base layer oily and soggy.
Store bars in an airtight container at room temperature for up to 3 days, or in the fridge for up to 5 days.
Nutrition info is based on 1 bar, assuming the pan is sliced into 9 equal bars.