Preheat the oven to 325 degrees F. Spray a 9x13-inch pan with nonstick spray.

In a large bowl, use an electric mixer to beat the 1½ cups (3 sticks) butter, the 3/4 cup packed light brown sugar, and the 3/4 cup granulated white sugar on medium speed until creamy, about 2–3 minutes. Add the 2 teaspoons vanilla extract and 3/4 teaspoon salt and beat until combined.

Reduce the mixer speed to low and add the 3½ cups all-purpose flour. Beat just until a smooth dough forms and no large pockets of dry flour remain. Scrape the bowl as needed.

Divide the dough into two equal portions. Wrap one half in plastic wrap and refrigerate for 30 minutes.

Press the other half of the dough evenly and firmly into the bottom of the prepared 9x13-inch pan to form the bottom crust.

Bake the crust in the preheated oven until it is just beginning to turn light brown around the edges, about 20 minutes. Remove the pan from the oven.

Evenly spread the 1½ cups Nutella (or other chocolate hazelnut spread) over the warm, partially baked crust.

Remove the chilled dough from the refrigerator and crumble it over the Nutella layer with your fingers or a fork, covering as much of the Nutella as desired to form a crumb topping.

Return the pan to the oven and bake until the topping is firm, crisp, and golden, about 30 minutes.

Transfer the pan to a wire rack and let the bars cool completely in the pan.

Lightly sprinkle powdered sugar over the cooled bars, then cut into bars and serve.