Preheat oven to 350 degrees + line a 13×9 baking dish with parchment paper.
To make the dough, in the bowl of a stand mixer beat together the butter, sugar, + salt on medium speed until creamy using the paddle attachment. beat in the vanilla.
Turn the mixer to low + slowly add 2 1/4 cups of the flour, beating just until the dough becomes smooth + the flour is incorporated.
Press 3/4 of the dough in the prepared baking dish. press the dough evenly to form the bottom layer. refrigerate while you prepare the crumb topping + filling.
Add the remaining 1/4 cup of flour to the remaining dough + beat together until coarse + crumbly. do not over mix.
For the caramel filling, place the butter, corn syrup, brown sugar, + condensed milk in a saucepan over medium low heat. bring to a simmer, whisking occasionally. when the mixture begins to boil, whisk frequently until it darkens + begins to get thicker, about 10 minutes. remove from the heat + allow to cool for 10 minutes.
Remove the pan from the refrigerator + spread on a layer of the nutella using an off-set spatula. you may need to warm the nutella in the microwave for a bit to soften it up enough to spread easily.
Pour the caramel filling over the nutella layer + spread evenly with the spatula.
Sprinkle on the crumb topping.
Bake for about 30 minutes or until the filling is a dark caramel color + lightly bubbling. cool on a wire rack for 15 minutes + then cut into squares.