Pistachio paste is made from dried pistachio nuts (seeds) ground into a paste. The paste may be prepared sweetened or unsweetened. Pistachio paste is used in baked goods, confections, gelato and even in some savory foods.
Look for pistachio paste in specialty grocery stores. Check your local Whole Foods Market, or Cost Plus World Market. Alternately you can purchase it online from a variety of sellers on Amazon.com: Pistachio paste
As of June, 2019, Christopher Kimball is selling a great Sicilian Pistachio Paste on his new Milk Street website.
If you don't have or can't find pistachio paste you can make your own. Here is an easy recipe for sweetened pistachio paste used in pastries and other baked goods like cookies:
Ingredients will vary depending on the brand:
Fiddyment Farms Pistachio Paste: - contains only ground roasted pistachio nuts.
Love'n Bake Pistachio Nut Paste: - sweetened variety includes: pistachios, sugar, canola oil, salt and natural flavor.