Place semisweet chocolate and bittersweet chocolate in a microwavable bowl. Microwave on HIGH 30 seconds. Stir well, and repeat for 30-second intervals until chocolate is completely melted and smooth, about 4 more times. (Note: If only a few lumps remain in the melted chocolate, microwave 15 seconds.) Stir vanilla into chocolate mixture until combined. Immediately spoon chocolate mixture evenly (about 1 1/2 tablespoons) into a 12- to 15-compartment silicone ice cube tray.