Batman #1: Heavy is the Head That Wears the Cowl by @Hinn_Raven.ao3 & @RenaRoo.ao3
There is no hiding from BATMAN, THE SILENT KNIGHT. Once born to assassins, raised to be a killer, CASSANDRA CAIN has taken an oath to not let anyone die on her watch. She has trained for the last several years to achieve the mantle of the BAT, but as soon as she is handed the cowl she is tested for it yet again. When the CLOWN PRINCE OF CRIME escapes from Arkham Asylum, there is a consensus that the new BATMAN is not ready. CASSANDRA disagrees. She must prove herself against the DARK KNIGHT's greatest foe, or possibly die trying! Cass!Bats vs Joker! I enjoyed this. And for real, of all the kids, she's the one who should take the cowl when Bruce retires.