First prepare the caramel. Add two tablespoon of sugar and one tablespoon of water to a pan. Set over high heat, so that the sugar dissolves completely. Cook for about 5 minutes, the caramel will get dark brown. Set aside. Set another pan over medium high heat, add the hazelnuts, toast them a bit and after about a minute, add the caramel. Stir well so that the hazelnuts become caramelized. Remove from the heat and place the caramelized hazelnuts onto a parchment paper to cool.
Add the butter to a saucepan. Set over a low heat and let it melt. Add the all purpose flour. Cook over a low heat for about 3 - 4 minutes, stirring constantly. Add 100ml milk, stir well, to get a thick mixture. Slowly add the rest of the milk, stirring constantly. Cook until the mixture starts to boil, stirring constantly. Add small chunks of dark chocolate. When the chocolate melts completely, set aside.
Add rum, pinch of sea salt, pinch of ground cardamom, pinch of ground allspice, pinch of ground cinnamon, pinch of ground ginger. Stir well, pour into your favorite hot chocolate cups, add a tablespoon of milk foam on top, sprinkle with ground cinnamon and add a few caramelized hazelnuts.