Combine all-purpose flour, cake flour, and salt in a large bowl.

Beat in eggs and water until a smooth and elastic dough forms (about 10 minutes of kneading).

Wrap the dough in plastic wrap and let it rest at room temperature for 30 minutes.

Boil peeled and cubed potatoes in salted water until very tender (about 20 minutes).

Strain the potatoes, reserving them.

While potatoes cook, heat vegetable oil in a large skillet over medium-high heat.

Add chopped onion, salt, and pepper, cooking until golden (about 6 minutes).

Add the cooked potatoes to the skillet with the onion and mash until smooth.

Season the filling with salt to taste.

Let the filling cool to warm.

Divide the dough in half, keeping one half wrapped to prevent drying.

Roll out the unwrapped dough on a floured surface into a 14-inch round.

Cut out rounds from the dough using a 2-3 inch round cutter.

Place a tablespoon of filling on one half of each dough round.

Moisten the edges of the rounds with water and fold into half-moons, sealing the edges.

Repeat with remaining dough and filling.

Boil the pierogi in batches in a large pot of boiling salted water until tender (about 12 minutes per batch).

Transfer cooked pierogi with a slotted spoon to an oiled baking sheet.

Melt butter in a large skillet over medium heat.

Brown the pierogi in batches, adding more butter as needed.

Serve the pierogi with sour cream and chives.