To make the custard filling:
- In a small bowl, whisk the custard powder and ½ cup of milk with a fork until smooth.
- Add this mixture to a medium saucepan and combine with the rest of the milk and the maple syrup.
- Cook over medium heat, whisking gently until the mixture thickens and begins to bubble.
- Take off the heat and immediately pour through a mesh sieve into the cooled tart shells.
- Sprinkle a pinch of ground nutmeg over each tart, then refrigerate until cold.