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:30487554). Recruited to the early elongation complex of RNA polymerase II (RNAPII) via interaction with POLR2A and mediates formation of m6A(m) co-transcriptionally (By similarity). a 5'-end (N(7)-methyl 5'-triphosphoguanosine)-(2'-O-methyladenosine) in mRNA + S-adenosyl-L-methionine = a 5'-end (N(7)-methyl 5'-triphosphoguanosine)-(N(6),2'-O-dimethyladenosine) in mRNA + H(+) + S-adenosyl-L-homocysteine Cap-specific adenosine methyltransferase activity is inhibited by zinc. Interacts with POLR2A; interacts with the phosphorylated C-terminal domain (CTD) of POLR2A. The WW domain is sufficient for direct and specific interaction with the phosphorylated CTD of RNAPII largest subunit. Belongs to the CAPAM family.