Which component can control the heat of a hot plate?
The most efficient way is to pulse the power to the heating element, this way the switch is either on or off and dissipates very little power (W = I^2 * R, so if switch off I = 0 so W = 0, and when switch is fully on, R = ~0 so W = ~0)  

For the switch you can use a MOSFET of suitable rating, and control it with a samll microcontroller (or some simple control circuit) which uses feedback from a temperature sensor and either just switches on/off above/below setpoint, or (better) performs a PI, or PID calculation (Proportional Integral Derivative) to keep the temperature more stable.