Energy Admixture( Tome and Blood: A Guidebook to Wizards and Sorcerers, p. 39)
You can modify a spell that uses one type of energy to mix in an equal amount of another type of energy.
Choose one type of energy: acid, cold, electricity, fire, or sonic. You can modify a spell with an energy designator to add an equal amount of the chosen type of energy. The altered spell works normally in all respects except the type of damage dealt (see sidebar). Thus, an acid fireball cast at 6th level deals 6d6 fire damage and 6d6 acid damage (roll each set of dice separately). The damage cap for a spell cast using this feat remains the same as the base spell but counts separately for each type of energy. So an acid fireball cast at 10th level or higher deals 10d6 fire damage and 10d6 acid damage. Even opposed types of energy, such as fire and cold, can be combined using this feat. An admixed spell uses up a spell slot four levels higher than the spell's actual level.
You can gain this feat multiple times, choosing a different type of energy each time. You can use Energy Admixture to further alter a spell that has already been modified with Energy Substitution. You can also use Energy Admixture to include your chosen energy type with a spell that already uses the same type, in effect doubling the damage dice.