Combine the sugar, cinnamon, cloves and allspice in a bowl. Place about 1/4 of the pumpkin into the bottom of a slow cooker, and sprinkle with 1/4 of the sugar mixture; repeat layers 3 more times. Cover the slow cooker, set it on Low, then cook for 8 hours or overnight. Stir once or twice during cooking. When the pumpkin is a dark, golden colour, place an immersion blender into the slow cooker and blend the pumpkin butter until smooth.
Sterilise the jars and lids in boiling water for at least 5 minutes. Pack the pumpkin butter into the hot, sterilised jars, filling the jars to within 5mm of the top. Run a knife or a thin spatula around the insides of the jars after they have been filled to remove any air bubbles. Wipe the rims of the jars with a moist paper towel to remove any food residue. Top with lids and screw on rings.
Place a rack in the bottom of a large stockpot and fill halfway with water. Bring to a boil over high heat, then carefully lower the jars into the pot using a holder. Leave a 5cm space between the jars. Pour in more boiling water if necessary until the water level is at least 3cm above the tops of the jars. Bring the water to a full boil, cover the pot and process for 10 to 15 minutes.
Remove the jars from the stockpot and place onto a cloth-covered or wood surface until cool. Once cool, press the top of each lid with a finger, ensuring that the seal is tight (lid does not move up or down at all). Store in a cool, dark area.