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 interval until the chocolate is melted and the consistency is smooth. This will take a total of 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.