TO MAKE THE CARAMEL: Combine cream and salt in small bowl; stir well to dissolve salt. Combine water and corn syrup in heavy-bottomed 2- to 3-quart saucepan; pour sugar into center of saucepan, taking care not to let sugar granules touch sides of pan. Gently stir with clean spatula to moisten sugar thoroughly. Cover and bring to boil over medium-high heat; cook, covered and without stirring, until sugar is completely dissolved and liquid is clear, 3 to 5 -minutes. Uncover and continue to cook, without stirring, until bubbles show faint golden color, 3 to 5 minutes more. Reduce heat to medium-low. Continue to cook (swirling occasionally) until caramel is light amber and registers about 360 degrees on candy or instant-read thermometer, 1 to 3 minutes longer. Remove saucepan from heat and carefully add cream to center of pan; stir with whisk or spatula (mixture will bubble and steam vigorously) until cream is fully incorporated and bubbling subsides. Stir in butter and vanilla until combined; transfer caramel to microwaveable measuring cup or bowl and set aside.
TO MAKE THE BROWNIES: Adjust oven rack to lower-middle position; heat oven to 325 degrees. Lightly spray 9-inch-square baking pan with nonstick cooking spray. Cut 14-inch length extra-wide heavy-duty foil; fold cut edges back to form 8 1/2-inch width. With folded sides face down, fit foil securely into bottom and up sides of baking pan, allowing excess to overhang pan sides. Spray foil with cooking spray.
Melt butter and bittersweet and unsweetened chocolates in medium heatproof bowl set over saucepan of barely simmering water, stirring occasionally, until smooth and combined; set aside to cool slightly. Meanwhile, whisk together flour and baking powder in small bowl; set aside. When chocolate has cooled slightly, whisk eggs in large bowl to combine; add sugar, salt, and vanilla and whisk until incorporated. Add melted chocolate mixture to egg mixture; whisk until homogenous. Add flour mixture; stir with rubber spatula until almost combined. Add chopped pecans and chocolate chips (if using); mix until incorporated and no flour streaks remain.
Distribute half of brownie batter in prepared baking pan, spreading in even layer. Drizzle scant 1/4 cup caramel over batter. Drop remaining batter in large mounds over caramel layer; spread evenly and into corners of pan with rubber spatula. Drizzle additional scant 1/4 cup caramel over top. Using tip of butter knife, swirl caramel and batter. Bake brownies until toothpick inserted into center comes out with only a few moist crumbs attached, 35 to 40 minutes. Cool brownies in pan on wire rack to room temperature, about 1 1/2 hours.
Heat remaining caramel (you should have about 3/4 cup) in microwave until warm and pourable but still thick (do not boil), 45 to 60 seconds, stirring once or twice; pour caramel over brownies. Using spatula, spread caramel to cover surface. Refrigerate brownies, uncovered, at least 2 hours.
Using foil extensions, lift brownies from baking pan, loosening sides with paring knife, if needed. Peel away and discard foil. Using chef's knife, cut brownies into 25 evenly sized squares. Press a pecan half onto surface of each brownie. Serve chilled or at room temperature.