Add hot cocoa mix, powdered sugar, and heavy cream to a large glass or mason jar.
Use a hand mixer fitted with 1 beater to beat the cocoa mixture together on medium speed until combined. Increase speed to high and mix until stiff peaks form.
Add the chocolate syrup and milk to a glass and stir to combine.
Spoon the whipped hot chocolate on top of the chocolate milk and top with a drizzle of chocolate sauce and mini marshmallows.
Notes
If you prefer to use warm milk, add milk to a small saucepan and place over medium heat. Cook until warmed, about 3 minutes, before pouring into glasses.