Line an 8x8-inch baking pan with a sheet of parchment paper large enough to fit up and over the edges (to help remove the fudge from the pan). Set aside.

Place a medium saucepan over medium-low heat. Add the sweetened condensed milk, butter, vanilla, and salt; stir to combine. Cook, stirring continuously, until the butter melts and the mixture is hot but not yet simmering or boiling. Remove the pan from the heat.

Pour the dark chocolate chips into the saucepan. Wait for a couple minutes, then stir until the chips have completely melted and everything is well combined.

Pour into the prepared pan, and use an offset spatula to smooth the top.

Use the back of a spoon or a butter knife to scoop a little bit of the marshmallow fluff. Drag the marshmallow fluff along the top of the fudge very lightly to create spooky streaks.

Sprinkle on the red sprinkles, then place the small and large candy eyeballs all over the top.

Let cool at room temperature until firm, about 3 hours. Refrigerating will help it to cool faster, but may cause the fudge to become a bit dry and crumbly.

When ready to serve, lift the fudge out of the pan and transfer to a cutting board to cut into small squares. Store in an airtight container at room temperature for up to 1 week.