Cap-specific adenosine methyltransferase that catalyzes formation of N(6),2'-O-dimethyladenosine cap (m6A(m)) by methylating the adenosine at the second transcribed position of capped mRNAs (PubMed:30467178, PubMed:30487554, PubMed:31279658, PubMed:31279659, PubMed:33428944). Recruited to the early elongation complex of RNA polymerase II (RNAPII) via interaction with POLR2A and mediates formation of m6A(m) co-transcriptionally (PubMed:30467178). {ECO:0000269|PubMed:30467178, ECO:0000269|PubMed:30487554, ECO:0000269|PubMed:31279658, ECO:0000269|PubMed:31279659, ECO:0000269|PubMed:33428944}. This is the function of ENSG00000100982 (PCIF1, phosphorylated CTD interacting factor 1).