Lepton asymmetries present in the early universe can source helical hypermagnetic fields through the chiral plasma instability. If these hypermagnetic fields survive until the electroweak phase transition, they source a contribution to the baryon asymmetry of the universe. In this talk I will explain how this phenomenon can, one the one hand, be used to set the to date strongest bounds on primordial lepton flavour asymmetries and on the other hand, can play a key role in viable baryogenesis models.