Mix the salt into the hamburger and refrigerate for one hour.
Preheat your sous vide machine to a temperature of 131 degrees Fahrenheit.
When the hour is up, divide the hamburger into four equal parts and make patties.
Sprinkle the pepper and garlic salt on top of the hamburger patties dividing it evenly between patties.
Place each hamburger patty into its own vacuum sealed bag.
Once the water has reached the correct temperature, place the vacuum sealed hamburgers into the water and cook for 45 minutes.
Once the 45 minutes is up, remove each hamburger from the vacuum sealed bag and set on a plate to rest.
Put the butter into a cast iron skillet, turn the burner onto medium high and allow the butter to melt.
Once the butter is melted, add the hamburger patties and cook for 30 seconds to 1 minute per side. You want each side to be nice and brown.
Remove the hamburger patties and place them on a plate lined with paper towels so the excess grease drains off. I usually lay a couple layers of paper towels on top as well and pat it down to absorb any grease on top of the meat.