Mix 8 oz White Chocolate with 1/2 cup Fat Free evaporated milk in a microwave safe bowl. Place in the micro and cook for 30 second on high, stir. repeat about 2 or 3 times. Mixing very well between heating. Stir till smooth. Do not over heat. Set aside to cool.
Cream 1 1/2 cups softened butter with 1 teaspoon vanilla extract and 1 teaspoon very fine salt. (I put a teaspoon of salt in my smoothy maker/blender and processed till a fine powder) Or use unflavored popcorn salt.
When butter mix is light and fluffly slowly add powdered sugar, 1 cup at a time mixing till smooth and creamy. towards end start adding cool melted white-choco/millk mixture. Beating till smooth.
Add more FF evaporated milk as needed a tablespoon at a time till you get it to your chosen consistency.