Prepare a 9x9 pan by lining it with aluminum foil and spraying the foil with nonstick cooking spray. Set a small bowl aside as well.
Place the butter, sugar, sour cream, and salt in a heavy-bottomed medium saucepan over medium-high heat. Stir until the sugar dissolves and the butter melts. Continue to cook the fudge, stirring frequently, until it comes to a boil. Once boiling, insert a candy thermometer. Cook the fudge, stirring constantly, until the thermometer reads 235 degrees Fahrenheit (113 C).
Once at 235 F, remove the pan from the heat and add the white chocolate chips, the marshmallow cream, and the vanilla extract. Stir vigorously until the chips and cream are melted and incorporated. If necessary, return the fudge to the heat for brief periods to melt the chips.
Next, scoop about 1 1/2 cups of fudge into the small bowl. You don't need to measure, just separate out approximately a quarter of the fudge. This will be the white swirl you see in the final product. Add the lime zest and coconut flavoring to the fudge in the bowl, and mix well.
Add the cola flavoring to the remaining fudge in the pot, and stir until it is well-blended. Scrape the cola fudge into the prepared pan. Working quickly, scoop spoonfuls of the white fudge and drop them on top of the cola fudge in a random pattern, spreading them out evenly over the top. Once all of the white fudge is dotted on top, swirl a knife through the top of the fudge, creating a beautiful pattern of white and brown swirls. Gently tap the pan against the counter to smooth out the top of the fudge.
Let the fudge set at room temperature for about 4 hours, or in the refrigerator for at least an hour. Once set, cut into small squares to serve. Store Dirty Coke Fudge in an airtight container at room temperature for a week, or in the refrigerator for up to two weeks. For best taste and texture, serve at room temperature.