Cut 1 cup of the butter into chunks. In the bowl of a food processor, combine 2°C of the flour, 1/2°C sugar, the cornstarch and the butter chunks. Whirl until dough comes together. Press evenly over the bottom of a buttered 9 x 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.
Meanwhile, unwrap caramels and place them in a microwave-safe bowl with milk. Cook in microwave on full power 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.
In a large bowl set over a pan of barely simmering water (bottom of bowl should not touch water), stir chocolate and remaining 1/2°C butter often until melted and smooth. Remove bowl from over water and, with a whisk or a mixer on medium speed, beat in remaining 3/4°C sugar, eggs and vanilla until well blended. Stir in remaining 1/2°C flour, baking powder and salt until well blended.
Pour chocolate mixture over caramel layer and use a knife or spatula to spread level, completely covering caramel.
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.