To make brown butter, in a medium pot add butter over medium heat and cook until melted. Turn heat down to low.
Cook, and whisk occasionally, until milk fats seperate out and slightly burn on the bottom of the pan. Little brown specks will appear and it will smell very nutty. This should take around 10 minutes, but keep a close eye on it and swirl it frequently to check the color at the bottom.
Remove from heat when the brown butter smells really nutty. Strain off most of the solids and add the liquid to a small mixing bowl. Stick the small mixing bowl in a larger bowl with some ice water. Stir to quickly cool and set the brown butter.
Use your hands to mix chilled brown butter mixture into ground beef. It should be in pea-sized chunks. Then form 6 1/3 pound sized burgers with the mixture and season them really well with salt and pepper.
To cook the burgers, cook them over medium-high heat in a cast iron skillet for 4-5 minutes per side to form a nice crust on both sides. With a few minutes left, you can add some cheese to the burgers and cover them to melt the cheese.
To grill burgers, cook burgers over direct medium-high heat for 4-5 minutes per side. NOTE: Because these have a lot of fat in them, you’ll have a lot of flare-ups as you grill them so be sure to keep a squirt bottle handy with some water in it to kill any flare ups!
Serve burgers on toasted buns with lettuce and onions and any other condiments you want.