In the spirit of answering the question, how to make the device pictured. You will need:
1 weatherproof electrical box
2 weatherproof box connectors
1 dimmer (the picture shows a slide dimmer, rotary would work as well)
1 dimmer cover (this is usually included with the dimmer)
4 yellow wire nuts
1 short 3-wire extension cord (12/3) - you could buy pieces to put this part together, but it will be cheaper to by the cord, even if you have to get a 25'
All of the parts should be available at a home improvement store. If you were to use this with your cooker, the dimmer needs to be rated for at least 1500 watts. That may be hard to find.
Cut the cord about 18" from each end (you could leave either end longer if desired, the picture appears to show about 18") Cut another piece of the cord about 6" and set aside.
Insert each of the cut ends into one of the box connectors about 6". Cut 3" of the outer jacket off of each and strip 1/2" of the insulation from each wire. Install the connectors into the box, one in each end.
Connect the white wires together with one of the wire nuts.
Remove the green wire from the 6" piece of cord and strip 1/2" of insulation from each end. Connect one end of this piece and the two green wires from the cord with another wire nut.
Connect each of the black wires from the dimmer to one of the wires from the cord. Doesn't matter which of the wires goes to which end of the cord.
Connect the end of the 6" piece of green wire to the grounding screw on the dimmer.
Install the dimmer in the box and install the cover.
Now, I think this had been a total waste of time, other than you now have a nice dimmer you can use for a light sometime.
Your cooker is designed for 120V. The dimmer works by reducing the voltage going to the appliance (usually a light). If you try to use the cooker on the dimmer, it probably will not heat properly. It may even burn out the heating elements.
If you watch an electric oven, you will note that it controls the temperature by turning the element on and off as needed. Your cooker should be the same way. Perhaps there is a kit you can buy to achieve the same result?