Line an 8x8-inch baking pan with parchment paper, leaving an overhang on the sides for easy removal, and set it aside.

In a medium heavy-bottomed saucepan over medium-low heat, combine the sweetened condensed milk, hot chocolate mix powder, and unsalted butter.

Stir the mixture constantly for about 3 to 4 minutes until the butter is completely melted and the hot chocolate powder is fully dissolved and smooth.

Add the semisweet chocolate chips to the saucepan, stirring continuously until the chips are completely melted and the mixture becomes thick and glossy, about 3 minutes.

Remove the saucepan from the heat, then quickly stir in the vanilla extract until fully incorporated.

Gently fold in 1 cup of the mini marshmallows, stirring just enough to distribute them without letting them melt completely into the warm chocolate.

Spread the mixture evenly into the prepared baking pan, smoothing the top with a spatula, and immediately press the remaining 1/2 cup of mini marshmallows into the surface.

Refrigerate the fudge for at least 2 hours until it is completely firm, then lift it out using the parchment paper and slice into small squares before serving.