Beat the butter with the salt and the vanilla sugar (on the highest setting) until the butter looks almost white. Then gradually add the sifted powdered sugar and stir in well. When an airy cream is created, it is colored. To do this, divide the cream into as many portions as you want to use colors. Then the colors are wrapped in cling film, one side should remain closed. With the open side down, put the wrapped cream in a piping bag and squirt the frosting onto the cupcakes. Done =)