Line an 8x8-inch baking pan with parchment paper and lightly grease it.
In a medium saucepan, combine chocolate chips and sweetened condensed milk over medium-low heat.
Stir constantly until the chocolate is completely melted and the mixture is smooth.
Remove from heat and stir in peppermint extract and green food coloring, if using.
Pour the mixture into the prepared pan and spread evenly.
Top with sprinkles or crushed peppermint, if desired.
Refrigerate for 2 hours or until firm.
Cut into squares and serve or store in an airtight container.