Combine the cream, the zest, cinnamon, nutmeg, and ginger in a medium saucepan and bring to a simmer over medium heat. Remove from the heat.
In a medium bowl, beat the egg yolks and sugar until pale and frothy, about 2 minutes. Slowly drizzle in 1/2 cup of the hot cream, whisking constantly. Return the mixture to warm milk and stirring constantly, cook over medium heat until thick, about 4 minutes. Remove from the heat and serve warm. (Or, to serve chilled, place in an ice bath to cool, then cover with plastic wrap, pressing down against the surface to prevent a skin from forming. Refrigerate until well chilled, about 2 hours.)
Yield: 2 cups