Peel and cut the pumpkin. Bring to a boil with water, cinnamon and vinegar. Let it cook until very soft.
Remove the pumpkin from the water. Mash with a fork and return to the pan. Add the sugar, cinnamon and vinegar. Bring to a boil over medium heat, stirring occasionally. Until reduced and thickened.
Transfer to a greased tray with butter, spread and let it cool and dry well at room temperature until it forms that crispy crust. Preferably let it rest overnight.