Prep: grease a 9x13 dish with butter and generously sprinkle with powdered sugar.
In a measuring cup add cold water and beef gelatin. Stir until it is well mixed if there are clumps that is fine, it will dissolve as it sits. Set to the side.
In a pot add the hot water, sugar, and salt and bring to a boil, stirring occasionally. Use a thermometer to ensure the mixture is brought to 240°F. When it reaches 240°F remove from the heat. If you do not have a thermometer look for the mixture to stick to your whisk as shown in the picture below. Once it sticks to the whisk it is ready.
Transfer the boiling mixture into your stand mixer bowl. (If you don't have a stand mixer you can use a large bowl and a hand mixer for this process.) To the bowl pour in the gelatin mixture. Be careful not to splash the mixture onto you it is extremely hot.
Mix on low for 1 minute until the gelatin mixture is thoroughly combined. Then mix on 6 (medium) for 1 minute. Next mix on 8 for 1 minute. Mix on 10 (high) for 3 minutes. Stop the mixer and add the vanilla. Mix on low for 10-20 seconds. Mix on 10 for 1 minute.
Pour the mixture immediately into your greased and powdered 9x13 dish and spread the mixture out as evenly as possible. Dust the top of the marshmallows with powdered sugar. Allow the marshmallows to set overnight or put them in the refrigerator to set for a minimum of 1 hour before cutting.
After the marshmallows have set overnight or in the refrigerator cut the marshmallows into even pieces. When you tear the marshmallows apart dip the uncoated sides in powdered sugar to coat to prevent sticking.