As has already been mentioned, the simplest way to just vary the power is to simply turn off the supply for a certain number of cycles:
100% Power = All cycles turned on;
50% Power = Half of the cycles on, half off;
10% Power = 10% of cycles on, 90% of cycles off;
etc, etc

Actually, it's probably easier to do on a half-cycle basis; eg, this is easily achieved by triggering a triac at the start of the 'ON' half-cycles



Note that this just varies the input power to the heater (just as your variac did) - it does not control the temperature.