For a sweeter sauce, use less cocoa powder or baking chocolate, or use semi-sweet baking chocolate.
To change up the flavor, add a pure extract such as vanilla, peppermint, orange, raspberry, almond, coffee, coconut, etc. Or stir in a spoonful of peanut butter, a little strong-brewed coffee, chopped nuts, or even a little liqueur. Add after the final step before transferring to jar.