Combine all ingredients in a heavy-bottomed saucepan.
Bring to a boil, then reduce the heat to low heat (otherwise the pumpkin mixture will sputter).
Cook for 20 minutes, stirring often, until the puree has become caramelized and dark brown, with a thick and shiny consistency. As long as you stir every couple minutes and use a heavy-bottomed pan, it shouldn’t burn.
Once sufficiently thickened, let the pumpkin butter cool completely before storing.
