In a medium-sized saucepan, add chocolate chips and brown sugar. Pour in the milk.
Place saucepan over medium heat on the stovetop.
Heat through slowly while whisking so the chocolate chips melt through evenly and won't stick to the base, about 3-4 minutes. Do not bring it to a boil or simmer.
Once steam is coming off the surface, remove from heat. If desired, whisk vigorously to create foam.
Whisk through rum, adding less or more if desired.
Pour into mugs.
Top with marshmallows and chocolate shavings or dust over cocoa powder.
Serve and Enjoy!
Notes
Dark rum: I like to use those miniature bottles of liquor, about 50ml/2oz, instead of purchasing a large bottle. That is unless you want to ;). Add more or less rum depending on your preference.
Make it dairy-free: Use your favorite dairy-free milk.
Store leftovers: Allow the hot chocolate to cool completely before transferring it to an airtight glass jar, such as a mason jar, or a sealed container. Store it in the refrigerator for 2-3 days.
Make it in advance: You can also prepare this hot chocolate ahead of time using the same storage method above.
Reheat: In a saucepan on the stovetop over low-medium heat. Be sure to whisk it well to recombine any ingredients that may have settled, heating it up to your preferred temperature.