Bring Brown's milk and cocoa powder to boil in a heavy saucepan. In a large bowl, whisk together eggs and sugar. Slowly pour milk mixture into eggs stirring constantly to avoid cooking the eggs. Pour mixture back into the saucepan, add cinnamon; cook over medium heat, stirring constantly until it coats the back of your spoon. Pour custard through a sieve (strainer) into a clean bowl, stir in Brown's cream, vanilla, and spiced rum.
Cool and chill for at least 4 hours and up to 24 hours. Top with whipped cream and a sprinkle of nutmeg if desired.