Prepare a square 8x8 square pan by lining it with aluminum foil or parchment paper and buttering.
Place sweetened condensed milk, and butter in the bowl of a double boiler (or a heat safe bowl over a medium saucepan with about an inch of water in the bottom) and heat over medium heat, stirring frequently until heated through and well mixed.
Add in both kinds of chocolate chips and continue heating until the chocolate chips are melted and well incorporated. Remove from the heat.
Add in vanilla and hot chocolate powder and stir until thoroughly mixed.
Pour the fudge mixture evenly into the prepared pan and immediately sprinkle with marshmallows.
Place in the refrigerator for at least 4 hours.
When ready to serve lift the fudge out of the pan with the foil and place on a cutting board.
With a sharp knife cut it into 16 small squares before serving.
Store leftover fudge refrigerated in an airtight container.