Baking Almost-Disaster

Last Sunday, I made these Lemon Squares from Bakerella. Maintenance never came to fix my oven - so typical - but I thought I had it figured out. Instead of baking them at 350 for 20 minutes, I put them in at 300 for 10 minutes. Guess what? They were still burnt! The next day, I bought an oven thermometer at Target.