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. 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.
To build: 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!