This is a long but excellent winter hike up onto the rolling plateau above Peterskopfl. The first stretch is a wide, well maintained path, which is very popular with hikers, and winds up through the woods along then right orthographic bank of the Kaisertal. It climbs at a steady 10% gradient up to Ritzalm, where many hikers stop. The next section is steeper, through the woods, and you arrive at the Vorderkaiserfeldenhutte, another popular stopping point.
Here we get into the real meat of the hike. This section is steeper, and switchbacks - after fresh snowfall the path can be hard to find. I suggest ignoring Naunspitze and Peterskopfl on your way up, and visiting them on your return, to ensure you have enough time to get to Einserkogel.
Peterskopfl appears clearly on your right, and the path is straight on. In winter, there are many choices of routes, and your main goal is not to fall off the cliffs on either side. Explore the many wonderful gullies and dolines, and the smaller side peaks which offer excellent views into the valleys below.
From Einserkogel you can see over the Pyramidenspitze and the formidable climb up Zwoelferkogel. This would make a great extension if you have the time, but be careful - the ascent of Zwoelferkogel is steep and difficult. When we tried it (25.3.19, no snow for a week, ~6 C) the snow was slushy and gave poor grip.
If you instead return, don't forget to check out Naunspitze and Peterskopfl, which both have excellent views. The way down is a bit hard on the knees, but you'll have the satisfaction of an excellent hike with exntesive snow shoeing in the winter wonderland at ~1900 m!