Node.JS, 141 bytes

f=s=>{b=new TextEncoder().encode(s),h='';for(_ of b){H=(_>>4)&0x0F,l=_&0x0F,h+=String.fromCharCode(H+(H<10?48:87),l+(l<10?48:87))};return h;}

Usage

f("Hello World!")

Returns

48656c6c6f20576f726c6421