In a medium bowl, whisk together the powdered sugar, butter, 1 tablespoon of milk, the vanilla and salt until well combined. Add additional milk as needed to achieve the desired consistency.
Whisk until smooth. Spread the icing onto the bellies of the cooled Totoro tarts. Pipe dots for the eyes. When the icing has dried, you can use melted chocolate to make the eyeballs, noses and decorate the bellies.