Prepare a 9x9-inch baking pan by lining it with parchment paper or greasing it.

In a medium saucepan, combine the semisweet chocolate chips and sweetened condensed milk.

Heat over low heat, stirring constantly until the chocolate is completely melted and smooth, which should take about 5 minutes.

Remove the saucepan from heat and stir in the mini marshmallows until fully incorporated.

If using nuts, fold them into the mixture now.

Mix in the vanilla extract until well combined.

Pour the fudge mixture into the prepared baking pan and spread it evenly with a spatula.

Allow the fudge to cool at room temperature for about 2 hours or refrigerate to speed up the cooling process.

Once set, lift the fudge out of the pan using the parchment paper, cut it into squares, and serve.