Preheat oven to 325 degrees. Line a buttered 8-inch square baking dish with foil or parchment paper, allowing a 2-inch overhang. Butter lining (excluding overhang); set aside.
Make batter: Put chocolate and butter in a medium heatproof bowl set over a pan of simmering water; stir until melted. Let cool slightly.
Whisk together flour, baking powder, and salt in another medium bowl; set aside.
Put sugar and eggs in the bowl of an electric mixer fitted with the whisk attachment, and beat on medium speed until pale and fluffy, about 4 minutes. Add chocolate mixture, milk, and vanilla, and mix until combined. Reduce speed to low. Add flour mixture; mix, scraping down sides of bowl as needed, until well combined.
Pour batter into prepared dish. Bake until a cake tester inserted into center of brownies comes out with a few crumbs but is not wet, 27 to 30 minutes. Let cool on a wire rack.
Make topping: Bring 1/3 cup water and the sugar to a boil in a medium saucepan over medium-high heat, stirring, until sugar has dissolved. When mixture comes to a boil, stop stirring, and wash down sides of pan with a wet pastry brush to prevent crystals from forming. Continue to cook, swirling pan occasionally, until medium amber, 5 to 7 minutes.
Remove from heat, and immediately add cream, vanilla, and salt. Gently stir with a wooden spoon or heatproof spatula until smooth. Add pecans; stir until caramel begins to cool and thickens slightly, about 1 minute.
Pour caramel over cool brownies; spread with an offset spatula. Refrigerate until cold, 30 minutes to 1 hour.
Let brownies stand at room temperature at least 15 minutes before serving. Lift out, and cut into 16 squares, wiping knife with a hot, damp cloth between each cut. Brownies can be refrigerated in an airtight container up to 2 days.