Voltage Potentiometer 120v
What makes you think reducing the voltage will help? It won't.

The temperature in a popcorn maker is regulated, most likely there is a simple bimetal thermoswitch which does a simple on-off regulation at a certain temperature. Reducing the power to the heating element will not reduce temperature but instead, slowing down the regulation only.

You have to find that thermoswitch and replace it with one of the same type but calibrated to another temperature.

After you have done that, it's safe to reduce the fan speed. The popcorn makers I know have a very small 12V fan. You could put a 9V voltage regulator into the fan supply, that would slow it down to about 50% speed.