Preheat the oven to 175°C(350°F). Grease and line an 8x8-inch (20x20 cm) square baking tin with parchment paper, leaving an overhang on the sides for easy removal.
Melt the butter and chocolate together in a heatproof bowl over a pan of simmering water (double boiler), or microwave in short bursts, stirring in between until smooth.
Add powdered sugar, light brown sugar, room temperature milk and vanilla to the melted chocolate mixture. Whisk well until fully combined and the sugar begins to dissolve.
Sift in the dry ingredients - flour, cocoa powder, baking powder, and salt. Stir until the batter is smooth and no streaks of flour remain.
Pour the brownie batter into the prepared pan and smooth the surface.
Bake for 25–30 minutes, or until the top is set and a toothpick inserted near the edges comes out with moist crumbs. Prepare pecan pie topping while brownies are baking.