My first try at baking a cookie "sheet" cake was epic fail- lowered the temp but after 50 min, it still didn't cook through (basically used a double recipe). Anyone have suggestions as to try to get a cooked product using this size pan? If this doesn't work, has anyone ever cooked 1 in a 12" tart pan? Is the recipe doubled for either of these pans?Thoughts, comments, suggestions are GREATLY appreciated!! TIA

Am I understanding correctly that you are baking a cookie in that size pan?   I don’t remember making any that exact size but have made many rounds & hearts over 12”   I cannot figure why it didn’t bake in that amount of time   What temp did you use?  Do you have an oven thermometer in there  & are you sure the oven is working right?   It’s been years since I’ve made any but as I remember it should take no more than 30-45 minuets  

Yes, that's what I was trying to do! I do keep a thermometer in the oven and baked (per recipe 15-18 min @ 350), then added another 25 min @ 325. I laid a piece of foil over top just to attempt to keep the top from getting to brown.  I've made 9 x 13 pans but grandson wanted BIG cookie this time ;-)

How thick is the batter?  Should be no more than about 1/2”   Pat it out w/your hand to within a 1/2” of the edge of the pan— it should spread to the edges as it bakes   That size might be hard to handle —to turn out of the pan;  consider making 2 smaller ones & butting them together on 1 board  

That's probaly was my issue- it wasn't as thick as the sheet pan but, I did spread all the way to the edges. I'll give a another try in the morning. I did line pan with parchment, so although not cooked all the way, it came out of pan nicely.  Truly appreciate your advice, and if round 2 doesn't work, he'll get 2 butted together!