Bun filled with yakisoba (fried noodles). 焼きそばパン。A type of Japanese-style savory bread.
Yakisoba are thick ramen-type noodles (not soba!) made from wheat flour. Usually they are stir-fried with bits of pork and vegetables and flavored with a thick sweet sauce. One of the favorite garnishes is benishoga, shredded pickled ginger, and you see that also often on the yakisoba buns.
The bun used is the same type as for hot dogs. The yakisoba is drier than usual, as otherwise the bun would soon be saturated with oil and sauce. This type of bun is generally available in convenience stores, but also in stalls at festivals. It is also popular among high-school students, because it pairs a low price with being quite filling.
The idea seems weird: baked noodles on bread... until you try it and see how delicious this is! One of the oldest, standard types of Japanese-style bread!