Place sugar, baking soda, Jello and milk in a large saucepan. Use one larger than 4 quarts if you have it.
Cook the Jello mixture over medium heat until it reaches 234℉. Stir frequently to keep it from sticking to the bottom and scorching. Be very careful as the mixture is hot and will get foamy.
Once the mixture reaches 234℉, remove it from the heat. Pour into a bowl and add the butter. Beat until the mixture thickens a bit.
Pour into a greased and lined 8-inch square pan. Chill until firm, at least a couple of hours.
Cut into squares. Store your fudge in an airtight container at room temperature for up to 2 weeks or freeze for longer storage.
Notes
Fudge is best stored in a cool, dry place. Refrigerating the fudge can dry it out and make it crumbly.Have fun trying different flavors of Jell-o with this recipe. Orange or cherry fudge would be tasty as well.