Preheat the oven to 375 degrees F (190 degrees C). Grease a 9-inch square pan.
Sift flour into a bowl. Rub in butter with your fingertips until the mixture resembles fine bread crumbs. Mix in brown sugar and shape the dough into a ball.
Press dough into the prepared pan. Prick all over with a fork.
Bake in the preheated oven until set and golden brown, about 20 minutes. Let cool.
Meanwhile, combine condensed milk, butter, and brown sugar in a nonstick saucepan over medium-low heat. Cook, stirring constantly, until caramel comes to a boil. Reduce heat and cook until golden and pulling away from the sides of the pan, 4 to 5 minutes.
Pour caramel over cooled crust and refrigerate until firm to the touch, about 1 hour.
Place chocolate on top of a double boiler over simmering water. Stir frequently, scraping down the sides with a rubber spatula to avoid scorching, until chocolate is melted, about 5 minutes.
Spread melted chocolate over cooled caramel. Refrigerate until chocolate hardens, about 15 minutes. Cut into bars. Yields 12 servings.