Peel, core, and cut the apples and lemon juice in a pot with a little water or apple juice. Cook with the lid on until apples are soft and pulpy. Allow to cool, then blend the apples until smooth
Fill the bottom pot of your double boiler or a heavy bottom pan with 1-2 inches of water. Place on high heat and bring it to a boil. Once the water begins to boil, reduce to low heat to keep the water at a simmer.
Place eggs, brown sugar, cinnamon, vanilla extract, and apple sauce, into the top pot of your double boiler. Using a silicone whisk, whisk until completely blended, then continue to whisk as the curd cooks for about 5 minutes.
Add the butter one at a time and whisk until melted for about 10 minutes. Remove the pan from the heat. Pour curd into a jar or bowl and place a piece of plastic wrap directly on top so it is touching the top of the curd. This will prevent skin from forming on top of the curd. The curd will continue to thicken as it cools. Once cool, the plastic wrap can be removed.
Refrigerate the curd for up to about 10 days.