Circuit or component to detect temperature change
I think a bimettalic element, a start switch and a relay is what you need. The relay will use one of its contacts to short the switch so that after pressing the switch, the relay becomes latched and powers the heating element. The bimettalic strip will disconnect  the relay coil when temperature is reached. You can only reactivate providing the temperature has dropped below the bimettalic switch's lower threshold AND you press the switch.