Prepare a large baking sheet with a piece of parchment or waxed paper on top. In a heavy bottom pan or Dutch oven, cook the butter, sugar, and evaporated milk together, stir constantly.
Cook until the butter and sugar have completely melted. Continuing to stir, and bring the mixture to a boil, cook until it reaches the soft ball stage (between 235 degrees and 245 degrees). This takes approximately 11 minutes.
Stir in some black gel food coloring (if you want it really dark black like coal) until you reach your desired shade of black. To the sugar mixture, stir in the marshmallow fluff, chocolate morsels, and vanilla extract until the chocolate is completely melted and combined.
Add additional black gel food coloring if desired. Pour the fudge onto the prepared baking sheet and spread evenly. Allow to cool completely at room temperature.
Lift the fudge off the baking sheet using the waxed paper. Cut into squares. Refrigerate the fudge in an airtight container between sheets of waxed paper so they don't stick.
Note: I made this fudge on a 11x15 baking sheet. A 9x13 baking pan can be used for thicker pieces.