In mixing bowl beat butter and shortening with low speed
add in condensed milk, salt and vanilla essence and beat with medium speed ( around 30minutes plus or until its light and very smooth) - if you make butter cream for piping (example for making butter cream rose)dont beat too long coz you need a slightly thicker butter icing mixture , so you can easily making your rose petals. *_^
Now you can add coloring to your butter cream 