Over low heat, melt butter until brown bits begin to form. This should take only a few minutes. You've just created brown butter, congrats!
Into the butter, add the two types of sugar and stir until they melt. Let cool before adding eggs (you don't want to end up with scrambled eggs!)
In a mixing bowl, whip egg until pale and frothy. This might take several minutes.
Add in the butter sugar mixture.
Fold in the dry ingredients: flour, cocoa powder and salt. Do not overmix, just stir through until you don't see streaks of flour anymore.
Finally, add in the chocolate squares.
Pour into a loaf tin lined with baking paper and bake at preheated 180 C for 20 minutes.
After removing from oven, lightly smash the tin on your counter top. This will help create a crackly top.
Allow to cool for at least one hour before eating. Enjoy!