Preheat oven to 350 degrees and line a 13x9 baking dish with parchment paper.
To make the dough, in the bowl of a stand mixer beat together the butter, sugar, and salt on medium speed until creamy using the paddle attachment. Beat in the vanilla.
Turn the mixer to low and slowly add 2 1/4 cups of the flour, beating just until the dough becomes smooth and the flour is incorporated.
Press 3/4s of the dough in the prepared baking dish. Press the dough evenly to form the bottom layer. Refrigerate while you prepare the crumb topping and filling.
Add the remaining 1/4 cup of flour to the remaining dough and beat together until coarse and crumbly. Do not over mix.
For the caramel filling, place the butter, corn syrup, brown sugar, and 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 and begins to get thicker, about 10 minutes. Remove from the heat and allow to cool for 10 minutes.
Remove the pan from the refrigerator and spread with the Nutella (I used 1/4 cup, but would add a bit more next time) 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 and spread evenly with the spatula.
Sprinkle on the crumb topping.
Bake for about 30 minutes or until the filling is a dark brown color and hot and bubbling. Cool on a wire rack for 15 minutes and then cut into squares.