How can i replace 12 ounces bittersweet chocolate?

im making this Chocolate Ganache Tart, but i RLY dont want all the calories in it, and most of them come from 12 ounces of bitter sweet chocolate. its used to thicken the Chocolate Ganache Tart, but i RLY dont need all thoes calories, is there something i can use to replace the chocolate or at least half of it? thanks:)