Line an 8x8 baking dish with parchment paper, leaving a slight overhang to help lift the fudge out of the pan. Binder clips can be used to fasten the top edge of the parchment paper to the dish, preventing it from folding over the fudge as it sets.

Lightly spray the parchment paper with nonstick cooking spray.

In a medium microwave-safe bowl, combine the semi sweet and milk chocolate chips, sweetened condensed milk, hot chocolate mix, and butter pats.

Microwave on full power for 1 minute, stir well, and continue microwaving in 45-second intervals until the chocolate is melted and completely smooth. This should take approximately 2.5 to 3 minutes.

Stir in the vanilla extract until completely incorporated.

Stir in 2 cups of the miniature marshmallows until well combined, reserving the remaining 2 cups for the topping.

Microwave the mixture for 20 to 30 seconds, then stir 2 to 3 times.

Pour the fudge into the prepared baking dish and use an offset spatula to spread it evenly.

Sprinkle the remaining miniature marshmallows over the surface of the fudge and gently press them down to adhere.

Cover the baking dish with plastic wrap and chill in the refrigerator for at least 2 hours, or until firmly set.

Once the fudge is set, remove any binder clips and use the parchment paper overhang to lift the fudge out of the pan onto a cutting board.

In a microwave-safe bowl, combine the chocolate chips for the drizzle and vegetable oil. Heat for 30 seconds, then stir until melted and smooth.

Drizzle the melted chocolate over the top of the marshmallows. Let the chocolate set completely before using a sharp knife to cut the fudge into 5x5 slices. Keep refrigerated until ready to serve.