Dragon-Style Twins

Double strike
Prowess (Whenever you cast a noncreature spell, this creature gets +1/+1 until end of turn.)

Pioneer Masters

"We are the flicker of the flame and its heat, the two sides of a single blade."

Khans of Tarkir Promos

Khans of Tarkir

Any spell you cast that doesn’t have the type creature will cause prowess to trigger. If a spell has multiple types, and one of those types is creature (such as an artifact creature), casting it won’t cause prowess to trigger. Playing a land also won’t cause prowess to trigger.

Once it triggers, prowess isn’t connected to the spell that caused it to trigger. If that spell is countered, prowess will still resolve.

Prowess goes on the stack on top of the spell that caused it to trigger. It will resolve before that spell.