*I used Hershey's Dark Cocoa here.
**Instead of using a microwave, you could use the double boiler method described in the cookbook. I didn't find there to be any discernible advantage, but that's just me.
Set a large heatproof bowl [or the top piece of a double boiler] over 1 inch of water in a large saucepan over medium heat. Add butter and chocolate to the bowl, stirring occasionally as they melt, until the sauce is smooth.
Whisk in the cocoa powder, then the brown sugar and salt. Continue whisking until the sugar dissolves completely and the mixture looks shiny. Add the eggs one at a time, ensuring each is fully incorporated before adding the next. The final mixture should be shiny and smooth.
Storage: If your tart lasts that long, it can be wrapped and stored in the refrigerator for about 5 days (bring it to room temp or eat cold) or pre-sliced and kept in the freezer for a couple months.