2½cupsall-purpose flour (spoon flour into a dry-measure cup and level off)
1tablespoonlight corn syrup
¼cupdark brown sugar, firmly packed
1 (14-ounce) can sweetened, condensed milk
One9 × 13 × 2-inch (23 × 33 × 5-cm) pan, buttered and lined fully (bottom and sides) with buttered parchment or foil
Set a rack in the lowest level of the oven and preheat to 350°F (180°C).
For the dough, beat the butter with the sugar and salt in an electric mixer with the paddle attachment on medium speed until soft and light, 2 to 3 minutes. Add the vanilla.
On the lowest speed, beat in 2¼cups of the Hour, 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 howl 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 ¼cup Hour into the remaining dough with your fingertips, so that it forms ⅛- to ¼-inch (3 to 6-mm) crumbs. Set aside at room temperature.
For the filling, in a medium saucepan bring the butter, corn syrup, brown sugar, 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 $0 minutes.
Cool in the pan on a rack until lukewarm, 15 to 20 minutes. Lift the slab of baked dough out of the pan and onto a cutting board before it has cooled completely. Cut the slab into 2-inch (5-cm) squares.
Keep these at room temperature if you are serving them within a day. If not, wrap and freeze them and make sure to defrost and bring to room temperature before serving.