Preheat oven to 350 degrees F. Line a light or medium-colored heavy-weight baking sheet with parchment paper or a silicone baking mat.Beat together butter and sugar with an electric mixer on medium-high speed until light and fluffy, about 2 to 3 minutes. Beat in egg yolk, vanilla and pistachio extract.Sift together flour, cocoa powder, and salt until evenly distributed. Add to mixing bowl with butter mixture, and mix on low speed until just incorporated and mixture comes together into a stiff dough. If your dough seems too crumbly, you can add 1-2 teaspoons of milk as needed to bring it together, however know your cookies might spread a bit more as a result.Roll dough into 1-inch balls and arrange on prepared baking sheet.For plain cookies, flatten balls slightly with your thumb or the back of a round 1/2 teaspoon measuring spoon, leaving an indentation in the center. You don’t want to press all the way down, just enough to form an indentation without the dough around it cracking. Dip the back of your measuring spoon in flour first so it doesn’t stick.For nut-coated cookies, lightly dip the ball of dough into reserved egg white, then roll between your palms until it’s thinly coated with egg white. Roll in chopped pistachios until evenly covered, then place on baking sheet, then press with your thumb or measuring spoon to indent.Bake cookies for 10 to 12 minutes or until centers are set and slightly puffed and edges are slightly darker in color. Remove baking sheet from oven.While the cookies are still warm, redefine the indentations with the same measuring spoon you used initially. They will have puffed a bit while baking, so doing this gives you room for your filling.While the cookies are cooling, make your filling. Place white chocolate in a small, microwave-safe bowl. Warm in the microwave on 50% power, stirring every 30 seconds, until melted and smooth.Stir in pistachio butter, powdered sugar, sea salt, pistachio extract and vanilla powder until smooth.Spoon 1/2 teaspoon of filling into the center of each cookie, taking care not to overflow the thumbprint (you might need slightly more or less filling per cookie depending on the size of your indentations).Let cool at room temperature for about 2 hours until set, or speed up the process by refrigerating the cookies for 10 to 15 minutes until centers are no longer shiny.Cookies will keep in an airtight container at room temperature or in the refrigerator for up to 5 days.