Combine chopped chocolate, butter, milk, and salt in a saucepan over low-heat.

Stir frequently until chocolate and butter are completely melted and mixture is smooth.

Remove chocolate mixture from the heat and pour into a large bowl. Add powdered sugar and vanilla extract and whisk until icing is glossy and smooth.

Allow the frosting to cool, stirring occasionally, until it is thick enough to spread or pipe over cake/cupcakes (how long this takes will vary greatly depending on many factors including how hot the chocolate mixture became, the temperature of your kitchen, etc.). The more you whisk the mixture the faster it will cool.