Heat sweetened condensed milk in a pan over medium-low heat just until thinned, about 4 minutes.
Stir in chocolate chips and stir until melted and smooth.
Remove fudge from heat and add in marshmallow fluff. Mix just to combine.
Spread fudge into prepared container. Sprinkle with sprinkles if using. Refrigerate until hardened, about 3 hours. Cut into pieces and serve.
Notes
You can substitute dark chocolate chips for the semi-sweet if you prefer. Milk chocolate would be too sweet. Nutrition information for estimation purposes only.