Bring large saucepan of water to boil. Meanwhile, mix potatoes, cream cheese, cheese, salt and pepper together.
Brush edges of won ton wrappers with egg white. Spoon 1 Tbsp. of potato mixture onto the center of each wrapper; fold in half. Press edges together to seal with a fork.
Add, in batches of 12, to boiling water in large saucepan; cook 2 to 3 min. or until pierogies float to surface. Drain in colander. Rinse with cold water; drain well.
Melt 1 Tbsp. butter in large nonstick skillet on medium-high heat. Add 12 pierogies; cook 2 to 3 minutes on each side or until golden brown on both sides. Remove from skillet; cover to keep warm. Repeat with remaining butter and pierogies. Serve with sour cream and chives.