Dough
8 ounces (2 sticks) unsalted butter, softened
½ cup sugar
¼ teaspoon salt
1 teaspoon vanilla extract
2½ cups all-purpose flour, divided
Filling
4 tablespoons unsalted butter
1 tablespoon light corn syrup
¼ cup dark brown sugar, firmly packed
1/2 teaspoon flaky sea salt (optional)
1 (14-ounce) can sweetened, condensed milk
Preheat oven to 350ºF. Line a 9x13x2-inch pan with parchment paper.
For the dough:
In an electric mixer with the paddle attachment on medium speed, beat the butter with the sugar and salt until soft and light, 2 to 3 minutes. Add the vanilla.
On the lowest speed, beat in 2¼ cups of the flour, scraping the bowl and paddle with a rubber spatula and continuing to mix just until the dough is smooth, and the flour has been absorbed.
Remove the bowl from the mixer and scrape ¾ of the dough into the prepared pan. Use the palm of your hand to press the dough down evenly, without compressing it too much. Chill the dough-lined pan. Work the remaining ¼ cup flour into the remaining dough with your fingertips, so that it forms 1/8- to 1/4-inch crumbs. Set aside at room temperature.
For the filling:
In a medium saucepan, bring the butter, corn syrup, brown sugar, optional salt and condensed milk to a simmer, stirring occasionally. Allow the mixture to boil gently, stirring often, until it starts to thicken and darken slightly, about 10 minutes. Pour into a stainless-steel bowl to cool for 5 minutes.
Remove the dough-lined pan from the refrigerator and scrape the cooled filling onto the dough, using a small offset spatula to spread the filling evenly on the dough. Scatter the crumb mixture on the filling.
Bake until the filling is bubbling gently and is a deep caramel color and the dough and crumb topping are baked through, about 30 minutes.
Cool in the pan on a wire rack until lukewarm, 15 to 20 minutes. Lift the slab of baked dough out of the pan and onto the cutting board before it has cooled completely. Cut the slab into 2-inch squares.