Stir together the milk, cream and cinnamon in a saucepan. Grate in 1/2 of the tonka bean (reserve the rest for garnish). Bring to a slow boil over medium heat. Take off the heat and allow the tonka and cinnamon to infuse the milk.
In a large bowl beat egg yolks and sugar until you can make thick ribbons when the whisk is lifted. Continue whisking while pouring in the milk and mix until the custard is smooth and silky. Discard the cinnamon quill.
Stir in the rum then refrigerate until evenly chilled or overnight.
Once cold, remove from the fridge and beat the egg whites to soft peaks in a large bowl. Gently whisk into eggnog until combined.
Pour evenly into glasses and garnish with freshly grated tonka before serving.