Line a 9x13 baking dish with foil and lightly spray with cooking spray. Set aside.
Combine granulated sugar, sour cream, butter and salt in a heavy 2 qt saucepan.
2 cups granulated sugar, ¾ cup sour cream, ½ cup unsalted butter, ½ teaspoon fine sea salt
Bring mixture to boil over medium heat, stirring frequently.
Continue cooking, stirring occasionally, to soft ball stage, 238°F. Use a candy thermometer like this one to make sure the fudge isn't over or undercooked. Remove from heat and stir in white chocolate chips until fully melted. This will take several minutes.
1 cup white chocolate chips
Stir in the marshmallow creme and vanilla extract until smooth and blended.
7 ounces marshmallow creme, 1 teaspoon clear vanilla extract
Stir in dried cranberries until evenly distributed.
5 ounces dried cranberries
Pour into the baking dish. Use an offset spatula to spread and smooth if need. Cool to room temperature.
Chill in refrigerator for several hours before cutting into squares and serving.