For an 8×8 pan, you can try 325°F and start checking at around 20 minutes, then every 5 minutes until done.

Yes you can 🙂

This recipe is baked in a 9×13 inch pan.

I used unsalted butter 🙂

Yes, you can use light brown sugar instead of dark brown sugar. The primary difference between light and dark brown sugar is the molasses content; dark brown sugar has more molasses, which gives it a richer flavor and darker color. Using light brown sugar will result in a slightly lighter color and milder molasses flavor, but it will still work well in the recipe.