6 tablespoons margarine, plus more for pan
6 ounces coarsely chopped good-quality semisweet chocolate
1/4 cup cocoa powder
3/4 cup flour
1/4 teaspoon baking powder
1/4 teaspoon kosher salt
1 cup sugar
2 large eggs
2 teaspoons pure vanilla extract
Preheat oven to 350 degrees. Line a greased 8-inch square baking pan with foil or parchment paper, allowing 2 inches to hang over sides. Set pan aside.
Put margarine, chocolate, and cocoa in a heatproof medium bowl set over a pan of simmering water; stir until margarine and chocolate are melted. Let cool slightly. You can also melt it in the microwave; melt at full power for 1 minute, stir and melt again for 1 minute longer. Stir to combine.
Whisk together flour, baking powder, and salt in a separate bowl; set aside.
Put sugar, eggs, and vanilla in the bowl of an electric mixer fitted with the whisk attachment, and beat on medium speed until pale, about 4 minutes. Add chocolate mixture; beat until combined. Add flour mixture; beat, scraping down sides of bowl, until well incorporated.
Pour batter into prepared pan; smooth top with a rubber spatula. Bake until a cake tester inserted into brownies (avoid center and edges) comes out with a few crumbs but is not wet, about 35 minutes. Let cool slightly in pan, about 15 minutes. Lift out brownies; let cool completely on a wire rack before cutting into squares.