In a small saucepan bring sweetened condensed milk to a simmer and and turn heat to low. Whisk in cocao powder and chocolate until melted. Whisk in water and increase heat to medium. Whisk milk into chocolate sauce until throughly incorporated. Continue to whisk until milk is heated through, being careful not to bring to a boil. After the mixture is warm, turn off the heat, and add liqueur of choice. Divide drinking chocolate into two mugs, top with slightly-whipped cream and garnish with cocao powder.