Here is an Instructable on how to build a DIY chocolate extruder for Ultimaker original. It is based on 20ml syringe with the inner diameter of 20mm. Syringe is covered with aluminum foil, then Kapton tape. The heater is made of NiCr wire coiled on covered syringe and itself covered with Kapton. To get uniform temperature on the syringe a layer is added of aluminum foil and Kapton outside the heater. Thermistor is NTC 100kOhm .
Detailed build log and chocolate preparation: