Reddit mentions of Whirl Admiration Pro-Fry Liquid Shortening Oil for Frying, 8 Pound

I think a lot of the answers here are close, but not close enough.

Metal: works fine, but a stainless steel pan is fine as well. I actually prefer stainless steel as cast iron can retain too much heat or keep going. However, if your surface doesn't have even heat distribution and your pan is even slightly warped, your burgers won't come out right. Restaurants cook over gas flame with stainless steel above it. Thus, the answer for you is to cook with a stainless steel pan over a gas stove.

Salt: most places put a lot of salt on to make things taste better, but to also accelerate the cooking process. Salt your patties. I don't suggest putting any other spices like pepper or whatever on until the very end, otherwise they can burn.

Fat: Using In-N-Out as the standard, I don't know what their fat content is, but I've been to the original stores where you walk up and can see the meat cooking 4' away from you and it's light pink. Very white. Plenty of fat. 90/10 is only good for making bolognese so you don't have to scoop much grease out of your sauce. I don't think 80/20 is fatty enough for a good burger. 85/15, 90/10 or 93/7 is even worse. Not only that, it's not going to be chuck all the time, but sirloin or even round. It's fine to blend beef and I've certainly ordered by own mix in the past (like chuck, tri tip and sirloin is a fave), but most places are just using ground chuck.

Handling: the more you mix up meat, the more it becomes a meatball. If you're using eggs, you're using the wrong technique. When I get the ground beef, I cut burger-sized pieces out of it and smash them directly onto the grill. I laugh at Smashburger because I've been doing that forever. One of the most renowned burger joints near me in Quebec (2 year winner of the Golden Fork for best poutine at the Festival du Poutine) has been smashing their burgers for 50-60 years. If you smash the burger down it will be ultimately tender. This was a technique I learned from my favourite restaurant before it closed.

Buns: your buns suck. Retail buns are shit. Even ones made by brands you know are different. You know why McDonald's buns are so unique? Rice flour. They are custom buns. The only restaurant-quality bun IMO are the ones from Pepperidge Farm. Another thing is liquid shortening. Your buns are "buttered" in something like this:

http://www.amazon.com/Whirl-Butter-Alternative-Gallon-Pack/dp/B00BUYTBAA

And then grilled. Otherwise, places that use toasted buns are putting them in a bun toaster. High heat, quickly. How? Broil in your oven like you had a Salamander.

Blasphemy: Another thing burger places do that you're told never to do is smash the meat while cooking. They do it to speed up the cooking process a bit. They don't turn the meat except once. You'll see cooks smash the burger right after they flip it. They're making sure it makes good contact.

Cheese: the processed cheese you get at the store isn't what the restaurants use. The closest to it is Kraft Deli Deluxe. Believe it or not, there actually is higher quality processed cheese. Another mistake is people use medium or above cheddar or melt cheddar. That just makes your burger a greasy mess. The sharper the cheese, the less creamy and more greasy it is. Always use thin-sliced mild cheddar and add the cheese as you remove the burger from the pan and let the leftover heat "sweat" the cheese. Melted cheddar on a burger completely changes the taste and consistency. Adding your tomato/lettuce/etc helps keep the cheese a bit cool as well.

Finally, if you don't mind the mess, my secret technique is to grill with sauce. In-N-Out will grill your burger with mustard. That's a good thing. Right at the end of the process, put mustard on the patty and flip it to the mustard side. I was friendly with the owner of a Fatburger who became my best friend when I told him I loved a chili burger with grilled mustard, chili, cheese and onions. "I got it", he said. I like to forego any salt when I cook and then grill each side with soy sauce at the end. Very savoury. My friends all rave about that. Grilling with a sauce before it burns but can caramelize with the meat is a great little science trick.

For maximum authenticity you're gonna have to get some Whirl

https://www.amazon.com/Whirl-Butter-Alternative-Gallon-Pack/dp/B00BUYTBAA