Line an 8x8 baking pan with parchment, leaving enough to hang over the edges.
Pour semisweet chocolate chips and sweetened condensed milk into a microwave-safe bowl. Microwave in 30-second intervals, stirring after each, until the chocolate is melted and smooth. This will take 2-3 minutes.
Stir in the vanilla extract and walnuts, then fold in the marshmallows.
Pour the mixture into the prepared pan, using a spatula to spread it evenly. Add sprinkles, if using.
Transfer to the refrigerator to chill until set, about 2 hours.
Using the excess parchment paper on the sides of the pan, lift the fudge out of the pan and set it onto a flat work surface. Cut into squares and serve.