I have a question about the recipe "Martha Stewart's Macaroni and Cheese" from Genius Recipes. What about the dish in the photograph? It is shallow. Is that better?
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Shallow means more space on top to get crusty. If you want a thicker, creamier (and less crunchy) version, I'm sure you can make this in a deeper dish.
It's hard to answer your question - it depends on the size of the vessel. A larger - by surface area and volume - baking container than a standard casserole dish will provide more area to spread the macaroni over, meaning it will cook faster and have more browned topping. If that's what you like (I know I prefer more crunchy topping) than go with that, but you may have to adjust the amount of bread topping since now you have a larger top to top, if you get my drift.
If your vessel is shallower but the dimensions are the same, you might end up with a big mess in your oven as your hot (and now, expanded) macaroni boils over the edge and burns.
If you go with a deeper dish that has a lower surface area, now the dish will heat slower and stay moist longer, which, consequently, will mean that you're most likely going to have to cook it longer and, perhaps, reduce the amount of bread topping. You'll also most likely end up with a moister product.
Also, depending on whether you go with a iron, aluminum, pyrex, or ceramic, you'll end up with different results. Generally metals conduct better than ceramics, which conduct better than pyrex. However, depending on the thickness of what you're using your results can vary.
I hope my complicated reply answered your simple question :)