You have a great capacity for spellcasting.
Prerequisite: Spellcasting class level 3
Benefit: You gain two additional spells slots for one spellcasting class you have levels in. You may never use this feat to have more spell slots at any level than you know at each level below it.
Special: You can gain Extra Spell Slots multiple times. Its effects stack.
Adventurer’s Handbook: Genius Guide Volume 1