In a small saucepan, heat the milk and chocolate over medium heat until simmering. Continue to heat, stirring occasionally until the chocolate melts, about 5 minutes. Meanwhile, fill a mug with hot water to warm its interior. Pour the water out of the mug and pour the genever, Grand Marnier and green Chartreuse into the mug. Pour in the hot cocoa. Garnish with whipped cream if desired, and serve.