1 stick (4 ounces / 113 g) unsalted butter, cold
2 large* eggs
2 to 3 tablespoons sugar
* If you're in the UK, that would be 2 medium eggs. See my British Conversions page for more details.
Note: Pour any leftover custard mixture into a custard cup, sprinkle with nutmeg and bake along with the tarts (and probably continue baking for a while after the tarts are done).