Melt butter and chocolate. Do this in a pot and stir until you have a smooth and uniform mixture.
Let the mixture cool for 5-10 minutes.
Whisk sugar and eggs together. They should become a fluffy eggnog.
Sift cocoa powder and all-purpose flour into the eggnog. Stir it gently and add salt.
Add the chocolate mixture and fold it in gently, I typically use a spatula for this.
Line a tart pan or springform pan with baking paper and pour the brownie batter into it.
Bake the cake for 25-30 minutes at 170 degrees Celsius (338 degrees Fahrenheit) fan-forced oven.
Let it cool, preferably on a wire rack.
Brownies benefit from standing a bit before eating, as they only get better. I think they taste even better the next day.
If you can't wait, you can of course eat it right away.
Serve it, for example, with vanilla ice cream.