Prepare the pan by lining an 8x8-inch square baking pan with parchment paper, allowing overhang for easy removal.

Melt the sweetened condensed milk and semi-sweet chocolate chips in a medium saucepan over low heat for 5 to 7 minutes, stirring continuously until smooth.

Remove from heat and stir in the vanilla extract until well incorporated.

Gently fold in the mini marshmallows without fully melting them, creating swirls.

Pour the mixture into the prepared pan, spread evenly, and swirl the top for presentation.

Cool at room temperature for 10 minutes, then refrigerate for at least 2 hours until set.

Lift out of the pan using parchment overhang and cut into 16 squares with a buttered knife.