In a large bowl, whisk eggs, water and salt until blended; gradually stir in flour. Transfer to a lightly floured surface; knead 10-12 times, forming a firm dough. Cover and let rest 10 minutes., For filling, in a small bowl, whisk egg, salt, sugar and pepper. Stir in cheese., Divide dough into 4 portions. On a lightly floured surface, roll each portion to 1/8-in. thickness; cut with a floured 4-in. round cookie cutter. Place 1 tablespoon filling in center of each circle. Moisten edges with water; fold in half and press edges to seal. Repeat with remaining dough and filling. , In a 6-qt. stockpot, bring water to a boil. Add pierogi in batches; reduce heat to a gentle simmer. Cook until pierogi float to the top and are tender, 2-3 minutes. Remove with a slotted spoon., In a large skillet, heat 1 tablespoon butter over medium-high heat. Add pierogi in batches; cook until golden brown, 1-3 minutes on each side, adding additional butter as necessary. If desired, serve with sour cream and chives.