How to Make It
Roughly chop the fat trimmings. Place fat trimmings and water in a large saucepan over medium-high; bring to a boil. Reduce heat to medium-low, and simmer until water is evaporated and fat has rendered. Strain fat through a fine-mesh strainer. Cool completely. (Store beef fat in sealed container in refrigerator up to a few months).
Stir together sugar, milk, cocoa, corn syrup, and salt in a 2-quart saucepan. Bring mixture to a boil over medium-high; cook until a candy thermometer registers 240°F. Remove mixture from heat; add 3 tablespoons of the beef fat, and let melt (do not stir). Let cool 10 to 15 minutes or until pan is cool to the touch. Stir in vanilla.
Line an 8-inch square metal pan with parchment paper, and coat parchment with some of the beef fat. Beat fudge mixture with an electric mixer at medium-high speed until mixture begins to lose its gloss and air is whipped in, 5 to 6 minutes. Working quickly, pour fudge into prepared pan. Sprinkle with sea salt; let cool 1 hour. Cut into 2-inch pieces.
Test kitchen tip: You can obtain beef fat scraps from your local butcher. From 2 pounds of scraps, rendered out about 1 cup of fat, with will keep for months, stored properly in the fridge. Try using your leftovers in other applications that would typically mean breaking out a stick of butter.