The body is an unevaluated chunk which you need to evaluate before you can concatenate it with your strings.
var curChunk = chunk.data.join(); // Capture anything in chunk prior to this helper
chunk.data = []; // Empty current chunk
var body = bodies.block(chunk).data.join() || '', // Evaluate block and make a string of it
.......
return chunk.write(curChunk + '<button class="' + btnClass + btnStyle + btnSize + '">' + body + '</button>'); // Prefix output with any earlier chunk contents and then build your tag.