Fill a medium sauce pan or pot with water.
Once water is boiling, place a heat resistant bowl (glass is best) on top of pot so that the water just hits the bottom bowl and lower heat to a simmer.
Take a fine mesh strainer put on top of another bowl and set aside. You will need this later after curd has set. It is important you have ready because your curd sets fast!
Place butter in bowl that is on top of bowling water to melt.
Once melted add maple syrup.
In separate bowl, beat eggs and yolk.
Pour in the fresh lemon juice and grated zest into bowl with butter and syrup.
Slowly add in eggs and whisk constantly.
Cook until the mixture has thickened and becomes jelly-like. Approximately 5-7 minutes but it can take longer.
With a pot holder lift bowl off the top of pot and pour curd into your strainer which is sitting on top of a clean bowl.
With a spatula or spoon press curd through the mesh leaving behind the zest and any egg that may have cooked.
Pour the curd into a jar or storage container and let cool before you put a top on.
Curd last for a couple weeks in the refrigerator.