Clarification from Apple, you can use an third-party USB-C power adapter that supports USB Power Delivery (USB-PD) (plus the official USB-C to lightning Cable of course)
https://support.apple.com/en-us/HT208137
Not from Apple you don’t, any third party will work..
What Apple fails to mention, is that (for fastest) charging you'd need a USB-C charger that supports 2 amps @ 14.5 watts, which is very rare, outside of Apple's own 29w MacBook charger.