Place potatoes in a medium heavy-bottomed saucepan. Add enough water to cover by 1-inch; bring to a boil over medium-high heat and cook for 15 minutes or until tender. Drain well.

Mash potatoes in a medium bowl with cheese, yogurt, 1 tablespoon butter and salt until smooth.

Bring a large pot of water to a boil. Working with a few wonton wrappers at a time, fill with a heaping teaspoonful of potato mixture. Brush edges with egg wash and fold over to create a half moon shape. Press edges with a fork to seal tightly and repeat with remaining wrappers and filling. Cook in batches in boiling water for 2 minutes or until the pierogi floats.

In a large skillet, heat remaining tablespoon of butter and olive oil over medium high heat. Transfer half the cooked pierogis to the skillet and sauté 2 to 4 minutes or until golden on each side. Repeat with remaining pierogis and serve with a side of apple sauce and/or sour cream for dipping.