In a large mixing bowl, cream together the softened unsalted butter and powdered sugar using an electric mixer on medium speed for about 2-3 minutes, until light and fluffy.

Incorporate the pistachio paste, egg yolk, vanilla extract, and optional almond extract into the creamed mixture and mix on low speed until well integrated.

In a separate bowl, whisk together the all-purpose flour and salt. Gradually add this dry mixture to the wet ingredients, stirring gently until just combined.

Portion the dough into small balls, about 1 inch in diameter, and place them on a lined baking sheet.

Preheat your oven to 350°F (175°C). Bake for 10-12 minutes, until edges are golden brown and centers appear slightly underbaked.

Once baked, let them cool on the baking sheet for about 5 minutes before transferring to a wire rack.

Prepare the cream filling by mixing additional pistachio paste, softened butter, and powdered sugar until smooth, adding milk if needed.

Take one cooled cookie and spread a generous amount of the pistachio cream filling on the flat side. Top with another cookie.

Dust the assembled cookies with powdered sugar and sprinkle with crushed pistachios for garnish.