Cook on medium-high heat while stirring constantly until the chocolate chips completely melt and the hot chocolate comes to a simmer. Then turn the heat off and serve immediately.
Divide the hot chocolate among mugs and top with whipped cream, chocolate shaving, or marshmallows. Make sure to take your first sips carefully as it will be pretty hot.
Notes
Make sure to stir constantly to avoid burning the chocolate on the bottom of the saucepan.
Use good quality chocolate chips. The better the quality of the chocolate, the better your hot chocolate will taste.
If you don't like super rich and chocolatey hot cocoa, you can thin it out with a little bit of extra milk.