Preheat the oven to 180°C (350°F). Line a square baking pan with baking paper.
Sieve together the flour, baking powder, and salt. Put aside.
Place the butter and chocolate in a metal bowl over a pot with simmering water. Let it melt until smooth stirring occasionally. Remove from the heat and add the cocoa powder, whisk until combined.
In a large bowl, mix together the eggs, granulated and brown sugars, oil and vanilla extract.
Add the chocolate mixture and whisk until smooth.
Gradually, two tablespoons at a time, add the flour to the batter, and mix with a spatula after each addition.
Pour the batter into the baking pan and bake for 23-28 min. The brownie should be set on the edges, glossy and cracked from the top, and the inserted toothpick should come out with loads of crumbs on it.
Let the brownie cool down in the baking pan. Remove from the pan by grabbing the baking paper from both sides and lifting the brownie up. Cut into the square pieces.
