DIAGNOSIS Unfortunately, that's a common design flaw with many of these budget electric Moka pots. The plastic handle is directly attached to the hot aluminum top chamber, so heat travels right into it.
FIX There's no true "fix," but there are workarounds. 1) Use a handle cover. A simple silicone trivet or even a folded, thick dish towel wrapped around the handle before you start brewing gives you a heat barrier. 2) Develop a two-handed technique when pouring. Use one hand with an oven mitt on the handle, and your other bare hand to support the base of the pot for control.
IF THAT DIDN'T WORK Check that you're not overfilling the top chamber. If coffee is bubbling up and contacting the underside of the plastic knob or lid, it transfers heat even faster. The coffee should only come about halfway up the inside of the top chamber.
HEADS UP Never try to cool the hot handle by running it under water. The sudden temperature shock can crack the plastic or cause it to separate from the metal. Let it cool down naturally on a heat-safe surface.