Combine the diced apples, sugar, lemon juice, and cinnamon in a large, heavy-bottomed pot. Stir, let sit 10 minutes, then cook over medium heat until the apples soften and the mixture starts to bubble, about 8 minutes.

Reduce the heat to medium-low and simmer, stirring every few minutes, until the jam is thick and leaves a trail when you drag a spatula through it, about 25-30 minutes.

Remove the pot from the heat and stir in the butter until melted and the jam is glossy.

Ladle the hot jam into clean jars, let cool completely on the counter, then refrigerate.