Place over medium-high heat to bring to a boil. Reduce the heat to the minimum, cover, and simmer for 20 minutes, until the apples are extremely soft (you should be able to break them down with a spoon).
Transfer half of the mixture to the container of a high-speed blender or a regular blender, and blend to a silky smooth consistency. (Make sure to start the blender on low speed and to hold down the blender’s cover while you do so because hot liquids will burst out.) Transfer the blended apple mixture to a large measuring cup. Blend the remaining apples and combine with the first batch.
Transfer the blended apples back to the skillet or pot. Bring to a slow simmer, then keep it over the lowest heat setting. Keep just enough heat so that a few bubbles come up to the top every second.
Simmer for about 1 hour and 20 minutes, whisking or stirring with a wooden spoon regularly, until the mixture has thickened considerably. The apple butter is finished cooking when it holds its shape after you swirl it with a spoon, or when you draw a line with a spoon on the bottom of the pan and it takes a few seconds before merging back together.
Transfer the apple butter to clean glass jars. Cover tightly and enjoy within two weeks.
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