2 cups flour
1-1/2 cups packed light brown sugar, divided
1 tsp salt, divided
1-1/2 cups cold, unsalted butter, divided
1-1/2 tsp vanilla, divided
1/4 cup light corn syrup
1, 14-oz can sweetened condensed milk
3/4 lb. milk or dark chocolate
Preheat oven to 325 degrees. Butter the bottom and sides of a 9x13 pan, layer the pan with parchment paper (the edges of the paper should hang over the top of the pan), and butter the paper. Make the shortbread by measuring the flour, 1/2 cup brown sugar and 1/2 tsp salt in a food processor and pulse just until mixed. Add 1 cup butter in chunks and pulse until roughly mixed. Sprinkle 1/2 tsp vanilla; process until a ball is formed. Press evenly over the bottom of the pan. Bake 35-40 min, turning the pan halfway through to brown evenly.
Make the caramel by melting the remaining 1/2 cup butter over low heat. Stir in the remaining 1 cup brown sugar, corn syrup and condensed milk. Raise heat to medium-low; bring to boil, stirring constantly for 10 min. Remove from heat; stir in 1/2 tsp salt and 1 tsp vanilla. Pour over the baked crust; let cool. Melt chocolate, then pour over the caramel layer. Cool completely in pan, then use parchment paper to lift the shortbread out of the pan. Cut into squares.