Grease a parchment lined 10 x 13.5 inch baking dish and preheat the oven to 335°F.
For the crust:
- Combine butter and sugar in a medium bowl and mix until light and fluffy, then add vanilla.
- In a separate bowl, sift together flour, powdered sugar, cornstarch, and salt then add to the butter and sugar.
- Place dough evenly into the baking dish and place in the refrigerator while the topping is being prepared.
For the topping:
- In a medium bowl, cream butter and brown sugar until light and fluffy.
- Add vanilla and eggs one at a time until blended. Combine flour, baking soda and powder, and salt in a medium bowl.
- In thirds, slowly add the dry mixture to the wet mixture. Fold in oats until fully incorporated. Place in the refrigerator to chill while assembling the bar.
- Pour chocolate hazelnut spread evenly over the prepared shortbread crust.
- Grab the cookie topping out of the refrigerator and crumble over the hazelnut spread until fully covered.
- Place in the oven and bake until the top is light brown, 25 minutes.
- Move to a cooling rack and let the bars set up for 1 hour.
- Cut into 5×4 squares and enjoy!