Can I also use a glass square pAn? And regular AP white flour?

Hi! You can certainly swap in all-purpose flour. The pan question is a bit trickier: The surface area of an 8-inch round pan (what the recipe calls for) is 50 square inches. What size is your square pan? The closest you could get would be a 7x7-inch (49 square inches), but if you have an 8x8 (64 square inches) or 9x9 (81 square inches), the cake is going to turn out significantly thinner. A glass pan also conducts heat differently than metal (more on that here: https://food52.com/blog/19554-why-the-material-of-your-loaf-pan-matters-more-than-you-think), so the baking time would be altered, even if the surface area was a perfect match. All of which to say...you could swap the pans, but it's hard to say how the cake will turn out. I still wouldn't butter the sides (this would prevent the cake from rising properly) and I would peek at the cake often to check on its progress. You also might not be able to unmold it as neatly as you can with the springform, but I find if you top anything with whipped cream, people don't care about the shape. If you give it a go, please let me know how it turns out!