Prepare the pan by lining an 8x8 inch baking pan with aluminum foil or parchment paper, leaving some overhang to make removal easier. Lightly spray with cooking spray.

In a double boiler, combine semi-sweet chocolate chips, milk chocolate chips, butter, sweetened condensed milk, and hot chocolate mix. Stir over medium heat for about 5-7 minutes until melted and smooth.

Once melted, add marshmallows and stir for about 1 minute, allowing them to soften slightly.

Remove from heat and mix in vanilla extract. Stir well to distribute evenly.

Pour the mixture into the prepared pan and spread evenly, pressing down gently to set.

Chill in the refrigerator for at least 4 hours until set.

Remove from the fridge and cut into squares using a sharp knife.

Melt remaining chocolate in a microwave-safe bowl and drizzle over the cut squares.