Charcoal, 3838 37 bytes

XＦ³²«Ｆ…·±ιιＦ…·±ιι«Ｊκλ›⁼№ＫＭX¹⁼ＫＫX»ＵＭＫＡXＪ³¹¦³¹XＦ³²«ＵＭＫＡXＦ⁶³Ｆ⁶³«Ｊκλ›⁼№ＫＭX¹⁼ＫＫX

Try it online!Try it online! Link is to verbose version of code. According to my comment on the Sandbox post I can do this in 37 bytes but unfortunately my computer rebooted to install updates and I didn't save my solution. Explanation:

XＪ³¹¦³¹X

Output the central X in the centre of the square.

Ｆ³²«

Grow the snowflake 31 times. (The firstlast loop has no effect since it only checks one squareis just used for its side-effect of replacing the -s with Xs.)

Ｆ…·±ιιＦ…·±ιι«ＵＭＫＡX

Check a larger square each time.Change all the (This is golfier than hardcoding-s to Xs.

Ｆ⁶³Ｆ⁶³«

Iterate over the sizesquare.)

Ｊκλ

Jump to each position in the square.

›⁼№ＫＭX¹⁼ＫＫX

If the cell does not yet have an X but it has one neighbouring X then print a -.

»ＵＭＫＡX

After checking the square, change all the -s to Xs.