In large bowl, beat butter and powdered sugar with an electric mixer on medium speed until creamy. Add in flour, salt, vanilla and pistachios until combined.
Shape dough into 1 1/4-inch balls. Place on a nonstick cookie sheet about 1 inch apart. Press thumb into center of each cookie (but not all the way through), to make an indentation.
Bake 14 to 16 minutes or until set but not browned. Quickly remake indentation with your thumb (or the end of a wooden spoon handle, if you can't stand the heat on your skin.)
Transfer cookies to cooling rack; cool for at least 20 minutes.
Fill each thumbprint with about 1/2 teaspoon of jam. Sprinkle the powdered over the top and enjoy!
Notes
In order to Freeze: make sure they are completely cooled (and wait to dust with the powdered sugar until you're ready to serve.) Put them in a sealable airtight container, in single layers, divided by waxed paper (this will prevent them from sticking to one another.) Alternatively, you can wait to fill them - freeze the cookies, defrost, and then fill them before serving. They can be frozen up to 1 month.