Slice flank steak across the grain into thin strips.
Combine marinade ingredients in a medium bowl.
Add beef slices; mix well.
Let stand 30 minutes.
Place egg roll skins on a flat surface with corners at top, bottom, left and right.
Place 1 tablespoon beef mixture near the bottom corner and spread it diagonally.
Fold bottom corner over filling and roll toward top corner, stopping a third of the way from top corner.
Fold left and right corners over filling.
Moisten inside of each corner with egg yolk and press to seal.
Egg roll should resemble a small envelope.
Heat 6 cups oil in a wok over high heat to 350°F (175°C).
Reduce heat to medium.
Carefully lower egg rolls into hot oil with a slotted metal spoon.
Deep-fry 2 or 3 at a time in hot oil until golden, about 1 minute on each side.
Remove from oil with slotted spoon.
Drain on paper towels.