In a medium saucepan, combine ¼ cup of water and 4 tablespoons of lemon juice. Add the chopped, peeled, and cored apples. Cover and cook over medium heat for about 20 minutes, stirring occasionally, until the apples are soft.

Remove from heat and let cool for 5 minutes. Mash the apples with a potato masher until smooth. Strain through a fine sieve for an ultra-smooth texture.

In a separate bowl, whisk together 2 whole eggs and 2 yolks until smooth, then set aside.

If using, mix 1 tablespoon of cornstarch with 2 tablespoons of water to create a slurry.

Add cubed butter and sugar to the apple purée. Stir in the whisked eggs and optional cornstarch slurry over low heat, mixing gently.

Increase heat slightly and cook the mixture for 15–20 minutes, stirring constantly until thick enough to coat the back of a spoon.

Remove from heat and strain the mixture again for a silkier curd. Let cool to room temperature, then transfer to a jar.

Cover and refrigerate for at least 2 hours to set before serving.