Butter
Butter is in 35 recipes and rarely as the main cooking fat. Its real job here is bun toasting: the milk solids brown at a much lower temperature than oil, so a bun face goes golden in under a minute and picks up a flavour that plain oil cannot give it. It also finishes onions — a knob added at the end of a long caramelisation carries the sweetness. Butter burns fast, which is why recipes that use it for the patty itself add it late rather than at the start.
Often goes with
Recipes with butter

Foie Gras Luxury Burger

The Heisenberg Burger

Paratha Smash Burger

McDonald's McSpicy (Copycat)

KFC Gravy Burger (Copycat)

French Onion Burgers

KFC Smoky Mountain BBQ Sandwich (Copycat)

Smash Burgers

Chipotle Chicken Burgers

Mushroom Swiss Burger

Homemade Smash Burgers

Spicy Chicken Burger with Beer Cheese Sauce

McDonald's Deluxe McCrispy (Copycat)

Viral Bacon Spiral Burger

McDonald's Teriyaki McDouble (Copycat)

McDonald's Bulgogi Burger (Copycat)

KFC Cheetos Chicken Sandwich (Copycat)

Duff Beer Sloppy Dog

The Crustburger

Buffalo Chicken Sliders

Chicken Parm Burgers

Ultimate Thanksgiving Leftovers Burger

Cuban-Style Sliders

Pickle Crustburger

Air Fryer Hamburgers

Cheeseburger Sliders

Fajita Cheeseburger

Good Morning Burger

McDonald's McAloo Tikki (Copycat)

Mushroom Swiss Smash Burgers with Truffle Sauce

Runny Egg Hashbrown Burger

KFC Chicken & Donuts Sandwich (Copycat)

ABQ Hot Chicken Sandwich

KFC Mac 'N Cheese Zinger (Copycat)

Chef John's Loco Moco