line slow cooker with parchment paper. Paper should be larger than your slow cooker so you can use the edges as a sort of "handle" to remove melted ingredients.
add all ingredients to lined slow cooker
12 ounces semi-sweet chocolate chips, 30 ounces milk chocolate chips, 16 ounces marshmallow fluff, 14 ounces sweetened condensed milk, 1 teaspoon pure vanilla extract, ½ cup butter
place lid on slow cooker and cook on high for 30 minutes
remove lid and stir ingredients to mix. Take care to stir all ingredients from bottom of slow cooker into the mix. Otherwise, the bottom will start to burn.
cover and cook for 15 more minutes.
Fully stir ingredients again to combine. Try to smooth out any unmelted chocolate chips while you are stirring. If your items are not fully melted and mixed, repeat the last two steps.
Once all ingredients are fully melted and mixed, turn off slow cooker and (carefully) remove ceramic insert from heating element.
The next step is to allow your fudge to cool and set by one of the following methods: 1. leave it as is in the ceramic insert until it cools (this will take many hours) 2. use the parchment paper sides to carefully lift the fudge from your slow cooker and quickly move it to another dish of similar size. (DO NOT place it on a flat surface). You can allow your fudge to harden either on either the counter, in the refrigerator (faster), or even in the freezer (fastest). The refrigerator method will take 3-4 hours while the freezer method can be done in as little as one hour.
if you would like to add any toppings to your fudge, add them while the fudge is still warm and soft so they will stick.
cut into squares and enjoy!