Line an 8x8 inch baking dish with parchment paper, leaving a slight overhang to help in lifting the fudge out of the pan. (Optional: Use small binder clips to fasten the parchment paper to the baking dish to prevent it from folding over.)

Lightly spray the parchment paper with nonstick cooking spray.

Add the semi-sweet and milk chocolate chips, sweetened condensed milk, hot chocolate mix, and sliced butter pats to a medium-sized microwave-safe bowl.

Heat at full power for 1 minute, then stir well. Continue to microwave in 45-second increments, stirring after each, until the chocolate is completely melted and smooth (this should take about 2.5 to 3 minutes total).

Stir in the vanilla extract until completely incorporated.

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

Microwave for an additional 20 to 30 seconds, then stir 2 to 3 times.

Pour the fudge into the prepared baking dish. Use an offset spatula to evenly spread the fudge mixture.

Sprinkle the reserved miniature marshmallows over the surface of the fudge. Gently press them down to ensure they adhere to the surface.

Cover the top of the baking dish with plastic wrap and chill in the refrigerator for 2 hours or until the fudge is 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.

For the chocolate drizzle, in a microwave-safe bowl, heat the 0.5 cup semi-sweet chocolate chips and vegetable oil for 30 seconds. Stir until melted and smooth.

Drizzle the melted chocolate over the tops of the miniature marshmallows. Allow the chocolate to set before using a sharp kitchen knife to cut the fudge into 5x5 slices. Keep refrigerated until ready to serve.