To the bowl of a stand mixer fitted with the whisk attachment, add the egg whites. Mix on medium speed for 1 minute until they start to foam.
Add cream of tartar and turn the speed to high. The cream of tartar will help prevent over-beating. When the whisk begins to leave ridges in the whites, check to see if the whites are stiff.
Remove the whisk and hold inverted. If the peak holds its shape and continues to point upward, you are done. If it bends back to the floor, return the whisk to the mixer and whisk for another 30 seconds or so.
One Last Thing
For the fluffiest stiff peaks, let your egg whites reach room temperature first.