Wash and peel the apples, remove the core, then cut into small pieces.
In a saucepan, add apples, cinnamon, lemon zest, lemon juice and 1 tablespoon water.
Stir everything and bring to a simmer for 15-20 minutes or until the apples have softened completely.
Remove from heat; transfer to a sieve, and use a spoon to press the apples through the sieve into another pan.
In a small cup, combine cornstarch with water, stir well until cornstarch is fully dissolved.
Pour the cornstarch mixture and brown sugar into the apple mixture, add vanilla, stir well.
Now, bring the mixture to a simmer for 1-2 minutes or until the sauce is thickened.
Remove from the heat, add the butter, stir until combined and let it cool.
Preferably choose tangy apples so that the sweetness of sugar is balanced out well.
If you are using sweet apples, cut down on the amount of sugar and add more lemon juice instead.
Make sure that the Curd cools down a little before you add the Butter.
If you are someone who loves an extra bit of sweetness with everything, you can add sweeteners like maple syrup, etc.
Make sure that the apples that you choose are firm and crisp.
Cover the Apple Curd with a cling wrap or a plastic wrap and press it gently onto the surface. This will avoid skin formation on the top.
When simmering the mixture, whisk it continuously to avoid burning.
You can substitute cinnamon with pumpkin pie spice, nutmeg, mixed spice, etc., to add slightly different flairs to the flavour of your dish.
You can use different kinds of sugar like white sugar, granulated sugar, etc., if brown sugar is not available.
Some people like to add eggs instead of corn starch to the mixture for a varied baked goods-like flavour profile and to help the Curd set better. If that is something that interests you, give it a try.
You can add fruits or even substitute apples with fruits like bananas, mangoes, berries, passion, etc. These additions/substitutions will definitely not disappoint you.
If you are not a fan of tang, you can leave the lemons out.