Hitting the ball with the bottom of the club face can result from several different swing faults
Focus on your left wrist at the top of the backswing and start of the downswing to get to impact properly, many players have a cupped left wrist which changes the angle of the clubface, this causes an open clubface and results in casting and manipulation during the last portion of the downswing
The golf swing does not pivot around the head or spine, the head moves in the backswing and both the right and left leg are pivot points in the swing