If a 6th-level Draconic Bloodline sorcerer with Elemental Affinity (fire) casts Green-Flame Blade, is only the first target's damage increased, or both?
The Elemental Affinity feature (PHB, p. 102) says:
[...] when you cast a spell that deals damage of the type associated with your draconic ancestry, add your Charisma modifier to that damage.
The description of the Green-Flame Blade cantrip (SCAG, p. 143) says:
[...] the target suffers the attack’s normal effects, and green fire leaps from the target to a different creature of your choice that you can see within 5 feet of it. The second creature takes fire damage equal to your spellcasting modifier.
[...] At 5th level, the melee attack deals an extra 1d8 fire damage to the target, and the fire damage to the second creature increases to 1d8 + your spellcasting ability modifier.