Line an 8x8 inch baking pan with parchment paper, leaving a bit of overhang on the sides for easy removal later.
In a medium saucepan over low heat, combine the semi-sweet chocolate chips and sweetened condensed milk. Stir gently until the chocolate chips are fully melted and the mixture is smooth.
Remove the saucepan from heat and stir in the unsweetened cocoa powder, vanilla extract, peppermint extract, and a pinch of salt until well combined.
Fold in the crushed peppermint candies, saving a tablespoon or so for topping if desired.
Pour the fudge mixture into the prepared baking pan, spreading it evenly with a spatula.
Sprinkle the top of the fudge with the reserved crushed peppermint candies and, if using, mini marshmallows.
Refrigerate the fudge for at least 2 hours or until set.
Once set, lift the fudge out of the pan using the parchment paper overhang and cut into squares.
Notes
Arrange the fudge squares on a festive plate and garnish with a few whole peppermint candies for a cheerful touch.