International audienceContext. Methanol, one of the simplest complex organic molecules in the interstellar medium, has been shown to be present and extended in cold environments such as starless cores. Studying the physical conditions at which CH3OH starts its efficient formation is important to understand the development of molecular complexity in star-forming regions. Aims: We aim to study methanol emission across several starless cores and investigate the physical conditions at which methanol starts to be efficiently formed, as well as how the physical structure of the cores and their surrounding environment affect its distribution. Methods: Methanol and C18O emission lines at 3 mm have been observed with the IRAM 30 m telescope within...