Line an 8×8 baking dish with parchment paper, make sure the sheet or paper is big enough to cover bottom and up the sides. Rub the parchment paper generously with butter. (If you do not wish to use butter, you can spray parchment paper with non-stick oil spray.)
Place white chocolate chips and marshmallow fluff into a mixing bowl.
Bring sweetened condensed milk and vanilla to simmer in a bottom-heavy sauce pot. (Make sure it does start to simmer before adding it to the bowl.)
Pour hot sweetened condensed milk into the mixing bowl with chocolate chips and marshmallow fluff and start stirring. Keep stirring until everything is mixed and smooth.
Pour chocolate mixture into the prepared baking dish and spread it evenly.
Drop a few spoonfuls of warm cranberry jam on top of fudge and gently swirl it into fudge using a thin spatula (or the other end of the spatula.)
Let fudge cool down to room temperature, cover the dish with plastic wrap, and place into the refrigerator to set.
Let it chill for 4-6 hours.
Take the parchment paper with fudge out of the pan and onto the cutting board. Pull down the edges of the parchment paper and cut fudge into squares using a sharp knife.