In large saucepan, heat sugar, butter, salt and cream until combined. Bring to a boil and stir continuously for 4 minutes.
Remove from heat. Quickly stir in white chocolate and marshmallow. Whisk in vigorously until smooth, be patient this will take a couple minutes.
Pour half of mixture into a parchment paper lined 13x9 baking dish. To remaining fudge, whisk in root beer concentrate. Stir until combined. Pour over white fudge. Using a knife, swirl the two together. Refrigerate 4 hours or overnight. Cut into bite size pieces and enjoy.