Google "PWM". You'll need to get a 25 amp SSR (solid state relay) as well and a power source for the PWM. Usually an old cell phone charger will work.
Hack the cell phone charger to power the PWM. The PWM connects to the control side of the SSR. One leg of the 220v goes through the SSR then on to the element.
Since PWM's typically don't go 100% on or 100% off, unless you get one with a potentiometer with a power switch built in, you'll want to put in a switch for the element to ensure you don't dry fire it.