01 - Pour the whole milk and heavy cream into a medium saucepan. Place over medium heat and warm the mixture until steaming, being careful not to bring it to a boil.
02 - Whisk the unsweetened cocoa powder and granulated sugar into the warm milk mixture. Continue whisking vigorously until both ingredients are completely dissolved and no lumps remain.
03 - Add the semi-sweet chocolate chips to the saucepan. Stir constantly with a whisk or wooden spoon until the chocolate has fully melted and the mixture is smooth and glossy.
04 - Remove the saucepan from the heat source. Stir in the vanilla extract until evenly distributed throughout the hot chocolate.
05 - Carefully pour the finished hot chocolate into two heat-safe mugs, dividing the liquid evenly between them.
06 - Generously top each mug with mini marshmallows and sprinkle with additional chocolate chips. Add whipped cream if desired. Serve immediately while hot.