Position a rack in the middle of the oven and preheat to 350 degrees F. Line a 13 by 9 by 2-inch-baking pan with parchment or wax paper, so that the edges of the paper come up the sides. Lightly butter the paper.
In a medium bowl, whisk together the flour, baking powder, and salt. Set aside.
Combine the semi-sweet and unsweetened chocolates and the butter in a heatproof medium bowl. Fill a medium saucepan with a couple inches of water and bring to a gentle simmer over medium heat. Turn off the heat. Set the bowl on the pan, over the heated water, to melt the chocolate gently. Whisk until smooth.
In a large bowl, whisk together the sugar, eggs, and vanilla. Stir the chocolate mixture into the egg mixture until evenly blended. Add the dry ingredients and mix just until blended. Take care not to over mix the batter. Transfer the batter to the prepared pan.
Bake until a toothpick inserted into the center of the brownies comes out coated with a fudge-y crumb, about 25 to 30 minutes. Cool the brownies in the pan on a rack.
To make the glaze, put the semi-sweet chocolate in a medium heatproof bowl. In a small saucepan, boil the heavy cream, and pour it over the chocolate. Let stand for 5 minutes, then stir until melted and smooth. Spread the glaze evenly over the brownies and let set at room temperature.
Lift the edges of the paper to take the brownies out of the pan. Cut the brownies into 24 squares. Serve.
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