Line an 8x8 dish with parchment. Spritz with cooking oil and set aside.
Combine the 1 ½ cups of semi-sweet chocolate chips, milk chocolate chips, sweetened condensed milk, hot chocolate mix, and butter in a medium saucepan. Cook over low heat until melted and smooth, stirring occasionally.
Remove from the heat and fold in half of the marshmallows and vanilla.
Transfer the fudge to the prepared dish and press it into a smooth even layer.
Top with the remaining marshmallows, pressing them gently into the fudge.
Refrigerate for 4-6 hours to set.
Melt the remaining semi-sweet chocolate chips in a microwave-safe bowl. Cook in 30-second increments, stirring in between each interval until smooth.
Drizzle over the top of the fudge.
Remove from the dish and cut into 1” cubes to serve.
Notes
Store in an airtight container for up to 10 days at room temperature.
Freeze in a freezer-safe container for up to 3 months. Thaw on the counter when ready to serve.4 hot chocolate packets equal about 1/2 cup.