Step #1 In a 3- to 4-quart pan over high heat, reduce heat, bring potatoes & 2 quarts water to a boil; cover, & let simmer until potatoes mash easily, about 15 mins.
Step #2 Drain & transfer to a bowl; mash with a potato masher until smooth.
Step #3 Meanwhile, about 10 mins (if onion begins to brown, in an 8- to 10-inch frying pan over med-heat/flame, stir onion & garlic in oil until onion is very limp, reduce heat to medium-low).
Step #4 Stir into potatoes, along with parsley, capers, cilantro, salt, & pepper.
Step #5 In a small bowl, beat egg yolk to mix; add to potato mixture & mix well.
Step #6 In another small bowl, beat egg white to mix.
Step #7 Cut each egg roll wrapper into four squares (keep covered with plastic wrap until ready to use).
Step #8 Place about 1 1/2 tsps potato mixture in the center of each square.
Step #9 Brush edges lightly with egg white.
Step #10 Fold each square diagonally over filling to form a triangle; pinch edges to seal.
Step #11 Pour 2 inches of oil into a heavy 5- to 6-quart pan over medium-high heat (or use a deep-fryer & follow manufacturer's recommendation for depth of oil).
Step #12 When oil reaches 375°, using a slotted spoon or a mesh basket & working in batches of 6 to 12 (pastries should fit easily in one layer), lower pastries into oil.
Step #13 Fry until golden brown, turning once, 3 to 5 mins total.
Step #14 Transfer to paper towel-lined 10- by 15-inch baking sheets in a 200° oven & keep warm up to 30 mins.
Step #15 Allow oil to return to 375° between batches.