Baking in rectangular or square pan

Hi Jenna! I've not yet done this, but KAF has a guide that looks pretty solid: https://www.kingarthurflour.com/blog/2019/10/28/how-to-make-sheet-cake-from-a-layer-cake-recipe

Looks like the batter for this recipe will fit in one 9x13x2" pan, and that baking time will actually increase by 30-40%.

Two 8-inch squares would be the same as 2 9-inch rounds. For a sheet pan, you may end up with slightly more batter than will comfortably fit in a 13x9. (Fill a ramekin or mug and make yourself a secret cupcake—we won't tell!). The proportions and temperature should remain the same, but you'll want to go by smell, look, and maybe a toothpick test rather than the time indicated for doneness.