Say hello to my little friend...
This may come as a shock to you, but not all ovens are the same or work the same way. If you have gas, electric, propane or whatever, your oven will work differently than most people's. Although recipes often use the same cooking temperatures -- 350, 400, etc. -- what you produce will vary depending on your own oven!
I distinctly remember my mother having to crank up our old, borderline antique oven approximately 45 minutes before she wanted to bake something, especially if she was making a cake or pastry of some sort. It took that long to get the oven up to proper and true 350. Now with modern ovens, often we set our oven temperatures digitally and the oven will beep when 350 or our desired temp is achieved. Shockingly, you'll find it probably wasn't so.
Have you noticed your oven seems to cook things faster than the recipe calls for? Have you tried roasting a chicken and the recipe says 450 for the first 30 minutes and the color of the chicken should start to turn golden, but after 15 you notice the top is actually burning? Or maybe the recipe says to cook the chicken a total of 1 1/2 hours and you take yours out, salivating at the aroma and cut in only to find the chicken isn't cooked all the way through? It's not you -- it's your oven! The temperature is off!
I don't know how, I don't know why, all I know is that it is.
I myself experienced this with my oven. Everything seemed to cook faster and the oven sometimes, especially with higher heats, just obliterated my foods. I mean the difference between leaving the cookies or cupcakes in an extra minute was perfectly done to black. It is that fast. Then finally I invested in an oven thermometer to properly gage the thing. It was shocking what I found...
First, even though my oven beeped when it reached 350 degrees, it actually wasn't at 350 degrees. It consistently showed 25 degrees less the first 20 minutes the oven was on. Meaning, I preheat my oven to 350 and once it beeps, I'd shove my food in and be in reality, cooking it at 325, not 350.
But then around 20 minutes into cooking the temperature jumped up 50 degrees and now it was cooking it 25 degrees over what my oven digitally was telling me. So even though 350 didn't change on the digital, in actuality it was now cooking my food at 375! My food basically had never even seen 350 during its entire time in the oven! Even though my oven digital read "350" the entire time!
I experimented with different temperatures and found the same exact thing happened. So if I was making cupcakes for example, I'd be starting them at 325 and then finishing them at 375 without me even knowing, which completely and totally explains why my foods tended to burn towards the end! After experimenting, I found my oven's true 350 was actually me setting it at 335 (!) and waiting about 20 minutes before I put food in.
Because using our ovens are so important in cooking, I highly recommend buying an oven thermometer. Especially if you do or plan to do a lot of baking or roasting, the different especially with baking of a 50 degree swing is huge. To test it out, simply hang or place your oven thermometer in the middle of your oven and gage:
(1) how long does it take to truly get the 350 degrees temperature, 375, etc.;
(2) what the true temperature is that you must manually set your oven to in order to achieve the desired cooking temperature;
(3) move the thermometer around the oven -- the middle, top and bottom, sides, up and down -- to see how evenly your oven cooks all around.
Sometimes ovens will have "cold spots" -- meaning half of the oven will cook faster than the other half, or perhaps heat conducts better towards the bottom rather than the top. This way you can move your oven racks according to what you're cooking. For example, if doing a cake and your bottom oven works harder than the middle or top, you may want to raise the oven rack higher and cook the cake further away from the bottom to prevent burning. Or perhaps the right side gets less heat than the left -- this means you'll have to remember to turn your food halfway between cooking to ensure even cooking on both sides of the dish.
You can purchase an oven thermometer at most grocery stores and other kitchen supply stores. They're inexpensive -- about $10 -- and will save you a lot of grief in the kitchen.