Beat in the egg and vanilla extract.
Some things to remember for baking almost any sheet pan cookie recipe: Your oven temperature and the type of cookie sheet you use will vary the baking times considerably. When they are golden brown around the edges is the time to remove them from the oven. Thinking they are a little underdone is probably a good thing. Experiment by baking only a couple of cookies at a time to see what the perfect baking time is for your oven.
Baking a few cookies at a time and noting the baking time each time will help you get the best result from your oven. I’ve seen different ovens have as much as 2 minutes difference in the bake time. I also highly recommend aluminum cookie sheets. In my experience the darker the metal in the pan the faster they conduct heat and are more likely to bake too quickly or burn. In my years of experience aluminum cookie sheets covered in parchment paper are the best.