First off, I have one thing to say about spices: I am not about to tell you how to season your chili; what you see here is what I do, but please adjust the spicing to suit your own tastes, whether you want to tone it down or spice it up.
Also, for the past couple of years, I have been using Penzeys medium-hot chili powder; if there's a Penzeys store near you or you order over the Internet from them, I highly recommend that blend.
To make the chili, you'll need a large heavy pot, like a Dutch oven.
Over medium-high heat, heat the oil in the pot and add the onion; saute for a couple of minutes, then add the garlic and mushrooms; saute for a couple more minutes.
Next, crumble the beef into the pot and stir well--the idea here is not to completely brown the beef, just take away the raw colour.
While beef is cooking, sprinkle with 1 tbsp of the chili powder; stir well.
After the beef has lost the raw red colour, spoon out any fat should you feel you need to.
Take one of the tins of drained tomatoes and dump into the pot, using a wooden spoon to break up the tomatoes into chunks.
Take the other tin of drained tomatoes and squeeze the tomatoes with your hand, to crush them; add these crushed tomatoes to the pot and combine well.
Stir in the well-drained kidney beans.
In a small bowl, using a fork or a small whisk, combine the brown sugar, flour and cocoa; set aside.
In a large measuring cup, stir together the tomato sauce and tomato paste, then stir or whisk in the cocoa mixture; when combined, add this mixture to the pot and stir well.
Now add the remaining 4 tbsp chili powder, oregano, cumin (if using, sometimes I leave it out), salt and pepper; stir well.
Let mixture start to bubble furiously, then turn the heat way down; after it's simmered for about 15 minutes, taste it; add more spices if you wish.
Let chili simmer, uncovered, for at least one hour, stirring occasionally (longer is better; best is to make it the day before you want it and let it simmer some more on the second day).
If you find it is cooking down too much or getting too thick for your tastes (this is not a thin chili), add some water or, even better, beef broth; best is a slug of beer should you happen to have some (I often don't).
Before serving, make sure you taste the chili and adjust the seasonings as required; I often find it needs more salt.
I serve this with sour cream on the table and usually some chopped raw green onion and shredded cheddar, plus a side of cornbread or cornbread muffins.