Peel the apples. Remove the core and cut the apples into small chunks.
Add apples to a saucepan.
Then add cinnamon, lemon zest, lemon juice & 1 tablespoon water.
Stir and bring to a simmer. Cover with a lid and let simmer gently until the apples have softened completely (about 15 minutes).
Once the apples have softened, remove the pan from heat.
Use a spatula to press the apples through a sieve into another pan. See photos/video for reference.
In a small bowl, combine cornstarch with 2 tablespoon of water. Stir until cornstarch is fully dissolved to create a slurry.
Add the slurry and sugar to the apple mixture. If desired, add vanilla here too. Stir well.
Bring the mixture to a gentle simmer. This will thicken the sauce and help it set.
Simmer until the curd reaches your desired consistency (in my case, this was 1-2 minutes).
Remove from heat and let it cool down slightly, then add butter if desired. Stir the butter through the mixture.