Place the butter, evaporated milk, sugar, and salt in a medium saucepan. Place over medium heat and bring to a rolling boil. Boil for 5 minutes, stirring constantly.
Take the pan off the heat and stir in the marshmallows, chocolate chips, and vanilla extract until the mixture is completely smooth. Pour the mixture into the prepared pan. Top with sprinkles or other candy as desired.
Refrigerate the pan for 2 hours, or until cool enough that the fudge is set. Cut into squares and serve.
Notes
Make sure that the marshmallows are soft and fresh (don't use marshmallows leftover from last Christmas, for example!). Dried out marshmallows will make the fudge grainy.You can substitute 14 large marshmallows for the mini marshmallows. Cut them into quarters with kitchen shears so that they melt easily into the chocolate mixture.Store the fudge in an airtight container in the refrigerator for up to 2 weeks. Place pieces of parchment paper or wax paper in between layers if you stack it. The fudge can be frozen, in an airtight container, for up to 2 months.