1. Cut 1 cup butter into chunks. In the bowl of a food processor, combine 2 cups flour, 1/2 cup sugar, the cornstarch, and the butter chunks. Whirl until dough comes together. Press evenly into a buttered 9- by 13-inch baking pan. Bake in a 325° oven until top feels dry and is just beginning to brown, about 25 minutes. Let cool completely.
2. Meanwhile, unwrap caramels and place in a microwave-safe bowl with milk. Cook in a microwave oven on full power (100%) for 1 minute. Stir mixture, then cook and stir at 30-second intervals until melted and smooth, about 3 minutes longer. Spread caramel mixture evenly over cooled crust and chill until firm, about 20 minutes.
3. In a large heatproof bowl set over a pan of barely simmering water (bottom of bowl should not touch water), stir chocolate and remaining 1/2 cup butter often until melted and smooth. Remove bowl from over water and, with a whisk or an electric mixer on medium speed, beat in remaining 3/4 cup sugar, eggs, and vanilla until well blended. Stir in remaining 1/2 cup flour, baking powder, and salt until well blended.
4. Pour chocolate mixture over caramel layer and use a knife or spatula to spread level, completely covering caramel.
5. Bake in a 350° oven until a wooden skewer inserted into the chocolate layer comes out clean, 35 to 40 minutes. Let cool completely, then cut into 24 bars.
Note: Nutritional analysis is per bar.