I Burned my Bottoms!

Dear Tricia, The last time I baked cookies, the bottoms were burned. I followed the recipe and baked them for how long it told me to, but the bottoms were almost black. What happened?

Ahhh.... the ultimate What the Bleep Happened question... I love fix-its! So, you burned your bottoms, huh? Fortunately, this is easy enough to solve, so I'll try to keep the sarcasm and bitterness to a low since this dilemma is quite common. But keep in mind that I'd usually mercilessly make fun of you for at least 2 paragraphs.
First and foremost, your oven temperature may be incorrect. I know... its mean for an oven to tell you its at 350 degrees when its actually at 410, but this is all too common. But much like Republicans, ovens are frequently wrong and you have to learn how to deal with them on their own terms. But we'll save that as a last ditch effort. On to the more common fix its.
Are you using an old-school cookie sheet? You know... that dark and rusty one that your grandmother gave you or the one that your mom got at a garage sale and gave you when you moved into your first apartment? Cookie sheets that are aluminum (and a large chunk of older or cheap ones are aluminum) conduct more heat than stainless steel ones. Its like your oven is a dance club and your aluminum cookie sheet is wearing an outfit made entirely by Ed Hardy: It yields a hot mess of terrible results. Stainless steel is the way to go. In the restaurant industry, we call them "sheet pans" and you can buy stainless steel half sheet pans that fit beautifully in your oven and give you incredible results with baking. You can get two for like 20 bucks at Sam's Club. And if you're still getting burned bottoms, use both sheet pans and double them up.
When you get your beautiful new half sheet pans, you'll also want to invest in some parchment paper. Its something else that we in the restaurant business swear by. You can find it in the baking aisle of your grocery store near the aluminum foil and it also comes rolled up in a little box just like foil. Tear off a sheet of parchment paper to line your sheet pan. You can make it stick by lightly spraying some Pam on the pan. Parchment paper doesn't usually need to be greased when you're baking cookies, so just put your dough straight on the parchment and that should give you perfect cookie color on the bottoms, free of a burned mess.
Have you mixed your cookie dough entirely? While you don't want to over mix your dough (that makes it spread out and get flat), making sure that the sugar is fully incorporated is super important because big chunks of sugar that have fallen to the bottom of the cookie dough will burn. If you ever see burned streaks on the bottom of the cookie, its most likely from sugar that wasn't mixed in all the way.
Also, make sure you're putting your cookies on the middle or top shelf of the oven, further away from the heating element. This was a confusing topic of debate in one of my older blogs, "Top or Bottom: Which is Hotter?", but for cookies, get on top. This helps many different... situations.
Back to your oven temperature being wrong: if none of the previous helpers are working for you, then your oven isn't calibrated correctly. You could try buying an oven thermometer to check the real temperature of the oven and the adjust the temperature from there. But if you're broke like me (and that goes for you, Mr.-Judgmental-Convenience-Store-Guy-Who-Gives-Me-Dirty-Looks-For-Buying-Cheap-Wine), turn it down about 20 degrees and give it a go. I think you'll find that very helpful.