18+ Boku wa Hajikko ga Suki

Noda Haruko has hatched a plot to get tasty lunches; she's put a love letter in a paper airplane and tossed it out into the world. By chance, it ends up in the hands of a lonely boy named Katou Seiji. Haruko confesses her "love" for him, and her hope that he'll become her husband and make her delicious lunches. Surprisingly, Seiji agrees, and becomes her boyfriend-chef. Seiji allows himself to be swept away by her childish enthusiasm, but he keeps trying to find ways to make their relationship more 'real'—by holding hands, kissing, or simply being alone together. Haruko and Seiji navigate their unusual relationship among their friends and classmates, who all have their share of comedic troubles!

(Source: MangaHelpers)