Stir together the milk, cream and St Nicholas Spekulaas in a saucepan. Bring to a slow boil over medium heat. Take off the heat.
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.
Stir in the rum then refrigerate until chilled, preferably 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 a sprinkle of St Nicholas Spekulaas before serving.
