Has anyone tried substituting condensed milk for the sugar? How much should I use? I was thinking of putting in the same amount as the cocoa powde...

I think I might know where you're going - but this brownie is perfect and spectacular as the recipe is written. It may look like a lot of sugar on paper, but I assure you complete balance of fudgey chocolate amazingness lurks deep within.

The method of this recipe basically requires that the sugar be white, and be beaten, hard, until shiny, because you are basically "making chocolate" out of cocoa - with butter and sugar - and if you omit the sugar and sweeten it with a liquid dairy, you could possibly end up with a soupy brownie, or worse yet - a gluey one. If you feel you must use sweetened condensed milk I offer you this opulent idea: turn that can into dulce de leche, take out 20% of the sugar, and once the batter is in the pan, swirl in dollops of dulce de leche. The brownie will grow around the DDL and, voilà! you will have one less item in your pantry. If you want a mocha brownie, I would suggest a shot of espresso or a dash of espresso powder, but if you add a lot of liquid to Cocoa Brownies, you will destroy its tenuous set.

(Disclaimer - this is one of my most favorite recipes of all time. It is really a miracle use for natural & Dutched cocoa alike.)

To be honest, I would find a recipe that calls for the ingredient you want to use rather than change Alice's so drastically. Is there a reason you want to use condensed milk? I think it's an ingredient more common in fudge.