Begin by lining an 8x8-inch square baking pan with parchment paper, ensuring that you leave some overhang to easily lift the fudge out later.

In a medium saucepan, combine the sweetened condensed milk and chocolate chips over low heat. Stir constantly for 5-7 minutes until the mixture becomes silky smooth and fully melted.

Once the chocolate base is melted, remove from heat and stir in the vanilla extract. Quickly fold in the mini marshmallows to create swirls without completely melting them.

Carefully pour the fudge mixture into the prepared baking pan and spread it evenly. Use a knife or spatula to gently drag through the mixture to create swirls.

Allow the fudge to sit at room temperature for 10 minutes, then refrigerate for at least 2 hours until completely firm.

Once set, lift the fudge out using the parchment paper and slice into 16 squares. Store any leftovers in an airtight container.