Benzopholes' Blistering

You create a Frostfire bolt at a point within range, where it explodes in a 5-foot-radius Sphere. Each creature in that Sphere must succeed on a Constitution saving throw or take 1d6 your choice of fire or cold damage and become vulnerable to cold or fire until the end of your next turn. 

Cantrip Upgrade. Create an additional bolt dealing 1d6 when you reach levels 5 (2d6), 11 (3d6), and 17 (4d6). Each bolt can target the same or a different target.