The origin of the hamburger as we know it is disputed, but there isn't actually a direct connection between its invention and the city of Hamburg. The wikipedia article has details.
I love Rentarou's synaesthesia here — the bread tasting like crunchy walnuts or bunny-shaped cookies. Perfect.
I could never be Rentarou. I don't even like half of these tastes, and honestly having them on bread would probably make them worse? Just goes to show how broad Rentarou's range goes. Truly the perfect boyfriend.