Chilling the burger before you cook it allows the outside to sear while the inside stays cold. It works best on a super-hot flat top where you can basically blacken each side then let the burger slow cook the rest of the way on a cooler part of the grille. If the meat is room temp when you cook it, then the juices will boil out all the way through the meat and you get a drier end product. You don’t have to freeze them, the fridge is typically good enough if it’s cold. I prefer to freezer them until they are stiff but not quite hockey-puck territory.
The other option is to smoke them very low & slow until the internal temp is where you want it then....BAM! .... throw the on a hot grille a sear the outside. That would be the reverse-sear method. I do that with prime rib on the bone at Christmas time - 250F oven until the meat hits 135-140F theb pull the roast, crank the broiler up to “Chernobyl Hot” , return the roast to the oven and sear the outside for 10mins. Makes the outside roast crispy brown to a depth of about 1/16” and the rest of the meat is a cool, pink medium rare.....mmmmmmmmmmmm, gaaaaahhhhh prime rib mmmmmmmm (that’s my Homer Simpson drool)
