1Heat the oven to 350°F and arrange a rack in the middle. Coat a 13-by-9-inch baking dish with butter; set aside.
2Combine the chocolate and measured butter in a small saucepan. Cook over low heat, stirring constantly, until melted, about 8 minutes. Remove from the heat and let cool slightly.
3Whisk the eggs, sugar, brown sugar, vanilla, and salt in a large bowl until the eggs are broken up and smooth. Add the chocolate mixture and stir to combine. Add the flour and baking powder and stir until just incorporated.
4Transfer the batter to the prepared dish and spread evenly. Bake until a toothpick inserted into the center comes out clean, about 25 minutes. Remove to a rack and cool completely.
For the caramel-pecan topping:
1Combine the sugar, water, and salt in a medium saucepan and bring to a boil over medium heat, stirring often until the sugar has completely dissolved. Continue to boil, swirling the pan occasionally (but not stirring), until the mixture turns a deep amber color, about 10 to 15 minutes.
2Remove from the heat, carefully pour in the cream and vanilla (the mixture will bubble up and steam), and stir until evenly combined.
3Stir the pecans into the caramel mixture and immediately drizzle over the brownies. Spread evenly to cover the brownies. Cool completely before cutting.