For caramel sauce, in a heavy small saucepan, combine whipping cream, 1/4 cup butter, brown sugar, and corn syrup. Bring to boiling over medium-high heat, whisking occasionally. Reduce heat to medium. Boil gently for 3 minutes more. Remove from heat; stir in 1/2 teaspoon vanilla. Let cool slightly. If desired, cover and chill for up to 2 weeks. Let stand at room temperature for 1 hour before serving.
Preheat oven to 325 degrees F. Lightly grease a 9x9x2-inch baking pan; set aside. In a small saucepan, combine bittersweet chocolate and 1/2 cup butter. Cook over low heat, stirring constantly, until melted and smooth. Remove from heat; cool slightly.
In a medium bowl, stir together flour, cocoa powder, baking powder, and salt; set aside. Set aside 25 pecan halves. Finely chop remaining pecans; set aside.
In a large bowl, combine the granulated sugar, eggs, and 1 teaspoon vanilla. Beat with an electric mixer on medium to high speed until pale and thickened, about 4 minutes. Beat in the melted chocolate mixture. Gently fold in the flour mixture and finely chopped pecans until combined.
Spread the batter evenly into prepared baking pan. Bake 35 to 45 minutes or until a toothpick inserted into the center comes out clean. Sprinkle chocolate pieces on top of the brownies. Loosely cover with foil and let stand 10 minutes. Spread melted chocolate evenly over the brownies. Arrange reserved pecan halves evenly on top of the brownies. Cool in pan on a wire rack. Serve brownies drizzled with caramel sauce. Makes 25 brownies.