Heat oven to 375°F. In food processor bowl with metal blade, place corned beef, cheese, sauerkraut and mustard. Cover; process until finely chopped.
If using crescent rolls: Unroll dough; separate into 4 rectangles. Press each into 7 1/2x5-inch rectangle, firmly pressing perforations to seal. Cut each rectangle into 6 (2 1/2-inch) squares. If using dough sheet: Unroll dough; cut into 4 rectangles. Press each into 7 1/2x5-inch rectangle. Cut each rectangle into 6 (2 1/2-inch) squares.
Place 1 teaspoon corned beef mixture on each square. Fold one corner to opposite corner, forming triangle and pressing edges to seal; place on ungreased cookie sheet. With fork, prick top of each to allow steam to escape. Brush tops with egg. Repeat with remaining half of dough and corned beef mixture.
Bake 9 to 14 minutes or until golden brown. Immediately remove from cookie sheet. Serve warm with dressing, if desired.