Preheat the oven to 180 degrees Celsius/ 360 degrees Fahrenheit. Grease a rectangular loose bottom tin (about 24x24 cm/ 9.4x9.4 inch) and line the bottom with baking paper. Another rectangular cake tin of similar size would be OK as well.
Mix the flour, cocoa powder, and baking powder. Set aside.
Melt butter: Cube the butter and melt slowly on very low heat.
In the meantime, finely chop the bittersweet chocolate.
Combine: Add the chocolate to the melted butter and stir until smooth. Don't let the mixture get too hot, remove it from the heat if it gets too hot.
Place the eggs, sugar, and salt in a bowl and mix with a hand mixer for about 5 minutes or until thick.
Immediately add the chocolate-butter mixture.
Add the flour mixture and mix only shortly, just enough to combine the ingredients.
Bake: Pour the mixture into the prepared tin and level. Bake for about 25 minutes. The brownies should remain rather gooey in the middle and don't get too dry. Let cool completely in the pan.