Preheat the oven to 350 degrees F. Spray the bottom and sides of a 9x13 inch pan with nonstick spray. Then line the bottom with a piece of parchment paper, allowing excess parchment paper to come up two sides, and spray again (lightly).
DOUGH
In a large bowl, use a hand mixer to combine the butter, sugar and salt. Beat at medium speed for 2 to 3 minutes, until the mixture is soft, light and combined well. Mix in the vanilla. Gradually mix in 2¼ cups of the flour, and beat until well incorporated. Scrape the sides, as needed.
Transfer ¾ of the dough to the prepared pan. Use clean hands to press the dough evenly into the bottom of the pan. Place the pan in the refrigerator while you make the topping. Work the remaining 1/4 cup flour into the remaining dough with your fingers so that it forms small crumbs. Set aside.
FILLING
In a medium saucepan, combine the filling ingredients. Heat over medium-heat, stirring occasionally. Let the mixture gently boil, stirring often, for about 10 minutes, until the mixture starts to thicken and darken slightly. Pour the mixture into a bowl to cool for at least 5 minutes.
Pour the cooled filling on top of the chilled dough, then scatter the crumb mixture evenly over the top.
Bake for about 30 minutes, until the filling bubbles gently and is a deep caramel color and the dough and crumb topping are baked through. Cool in the pan for at least 15 to 20 minutes.
Cut the sides of the pan to loosen the bars. Then use the parchment paper to lift the bars out of the pan and transfer to a cutting board. Cut the slab of bars into 2-inch squares.
Notes
These bars freeze well. Just wrap them well, and freeze for up to 2 months.