Counter Spell

You may Counterattack
when a creature within your wielded melee weapon’s reach
casts a spell by spending an action or as part of an action (ex: Dual Cast, Spellsword, Offensive Support).

You attack after the spell’s effect.
(Spells which move you or the caster out of range or deny your ability to use Counter Spell cannot be Counter Spelled.)
(If the spell is Counterspelled, you may still use this ability on that spell, and your attack happens after the spell is countered.)

Spell attacks only trigger one Counterattack (Not one because it is a spell being cast and another because it missed, or hit with Trade Blows).