Line an 8x8-inch square baking dish with parchment paper, allowing some overhang for easy removal later.
In a medium saucepan over low heat, combine the semi-sweet chocolate chips, sweetened condensed milk, and unsalted butter. Stir continuously until the chocolate and butter have completely melted and the mixture is smooth.
Remove the saucepan from heat and stir in the vanilla extract until well combined.
Gently fold in the miniature marshmallows and chopped walnuts (if using), ensuring the marshmallows are well-distributed throughout the chocolate mixture.
Pour half of the fudge mixture into the prepared baking dish, spreading it evenly across the bottom.
Add a spoonful of the remaining fudge mixture on top and use a knife to swirl it gently, creating marbled patterns.
Sprinkle colorful sprinkles on top for a fun finishing touch.
Place the fudge in the refrigerator for 2-3 hours or until set.
Once set, lift the fudge out of the dish using the parchment paper, and cut into squares.
Notes
Chopped walnuts are optional. Use parchment paper for easy removal.