1In a medium saucepan, combine the whole milk, desiccated coconut, and lemon zest. Warm over medium heat until steaming but not boiling.
Remove the saucepan from the heat and let the coconut and zest infuse in the milk for 5 minutes.
In a mixing bowl, whisk together the sugar, cornstarch, and eggs until smooth and pale.
Add the plain curd and lemon juice to the egg mixture, then whisk until fully combined.
Slowly pour the warm milk mixture into the egg mixture while whisking constantly to temper the eggs and prevent curdling.
Return the mixture to the saucepan and cook over low to medium-low heat, stirring continuously with a whisk or spatula.
Cook for 5 to 8 minutes, until the custard thickens enough to coat the back of a spoon. Do not let it boil.
Remove from the heat and whisk for 30 seconds to make the custard smooth and glossy.
Divide the custard evenly into serving cups or ramekins.
Let the custard cups cool at room temperature for 15 minutes, then refrigerate for at least 2 hours until fully chilled and set.
Serve cold, optionally garnished with a little extra lemon zest or coconut if desired.