Butterflied Strings Shortest Character Solution

The shortest character solution for Butterflied Strings on Wolfram Challenges according to the CODE CHARACTERS functionality is 40. I have a solution with 42 characters, namely ButterflyString@x_:= x~~StringReverse@x

As a reminder, the CODE CHARACTERS functionality is "Number of Characters Appearing in the InputForm of the code"

However, I can't reduce it to 40. Any suggestions?