Finely chop the semi-sweet chocolate and set aside.
Pour the milk into a medium saucepan and place over medium-low heat.
Add cocoa powder and whisk thoroughly.
Add brown sugar and continue whisking until dissolved.
Whisk in ginger, cinnamon, nutmeg, and salt.
Add the molasses and whisk until completely incorporated.
Heat gently until hot and steaming, stirring frequently. Do not rapidly boil.
Reduce the heat to low and add the chopped chocolate.
Whisk until the chocolate has completely melted and the drink is smooth.
Remove from heat and stir in vanilla.
Divide marshmallows, whipped cream, crushed gingerbread cookies, mini chocolate chips, and sprinkles into separate serving bowls.
Adults should pour warm hot chocolate into mugs for younger children.
Let everyone customize their mug with their favorite toppings.
Serve warm.