Line an 8x8 or 9x9 square pan with parchment paper for foil sprayed with cooking spray.
Assemble all your ingredients and measure everything out, so that it's all ready to go. Have a 4-cup measuring cup ready to split the mixture later on, to get the marble effect.
Bring sugar, butter and milk to a full rolling boil in a medium sized saucepan. A "full rolling boil" means that the boil continues even when you stir. Once it begins this type of boil, set the timer for 4 minutes, 30 seconds. You can use a candy thermometer and boil it until 234 degrees is reached, but really, setting a timer to 4-5 minutes works great too.
Now, this next part goes quickly, so make sure you have everything measured out and ready.
Remove pot from heat and add in the marshmallow creme. It's sticky stuff, so I like to spray a rubber scraper with cooking spray prior to scooping it out. Stir it into the butter-sugar-milk mixture until it all melts.
Add in the teaspoon of vanilla.
Once the marshmallow has melted and is smooth, pour out 2.5 cups into the 4 cup measuring cup you set aside.
Quickly add the vanilla chocolate chips to one batch, and the semi-sweet chocolate chips to the other.
To the semi-sweet mixture, also add 1 rounded teaspoon of cinnamon.
To the vanilla mixture, add in a pinch or two of salt. Stir each separately until smooth and blended. You'll want to work quickly- at this point the fudge is starting to set up! If you have another set of hands in the kitchen, recruit them to stir!
Once both mixtures are melted and smooth, you'll drop large spoonfuls of the mixture onto your pan.
Once you've used all the chocolate, take a knife and just quickly run lines through the mixture to swirl the two flavors together a bit.
Sprinkle the top with additional cinnamon. Let sit until firm.
Cut into 1" squares and ENJOY.