Is it possible to write on top of a buffer?

buff := bytes.NewBuffer(somebytes)

How to write on top of buff? Currently I'm creating a new buffer. Is this the right way?

newBuff := bytes.NewBuffer(otherbytes) 
newBuff.ReadFrom(buff)

bytes.NewBuffer() returns a *Buffer. *Buffer implements io.Writer (and io.Reader) so you can simply write to it by calling its Write() or WriteString() methods.

Example:

somebytes := []byte("abc")
buff := bytes.NewBuffer(somebytes)
buff.Write([]byte("def"))
fmt.Println(buff)

Output as expected (try it on the Go Playground):

abcdef

If you want to start with an empty buffer, you can simply create an empty Buffer struct (and take its address):

buff := &bytes.Buffer{}

If you want to "overwrite" the current content of the buffer, you can use the Buffer.Reset() method or the equivalent Buffer.Truncate(0) call.

Note that resetting or truncating the buffer will throw away the content (or only a part of it in case of Truncate(). But the allocated buffer (byte slice) in the background is kept and reused.

Note:

What you really want to do is not possible directly: just imagine if you want to insert some data in front of an existing content, the existing content would have to be shifted every time you write / insert something in front of it. This is not really efficient.

Instead create your body in a Buffer. Once it's done, you will know what your header will be. Create the header in another Buffer, and when it's done, copy (write) the body (from the first Buffer) into the second already containing the header.

Or if you don't need to store the whole data, you don't need to create a 2nd Buffer for the header. Once the body is ready, write the header to your output, and then write the body from the Buffer.