A friend suggested buying an oven thermometer to see if her oven was heating accurately. Turns out theoven was running about 75 degrees higher than it should. Hence the burnt cookies. Over time your oven can slip out of calibration and the temperature setting may no longer match the actual temperature inside your oven.
Buying an oven thermometer at the supermarket will be the best ten bucks you’ll spend. Place the thermometer on the center rack of your oven, turn the oven on and wait to see if your temps add up.