Combine flour and cocoa powder in medium bowl and set aside.

In bowl of an electric mixer fitted with paddle attachment, combine butter, sugar, and salt. Beat on medium-high speed until light and fluffy, scraping bowl as needed. Add melted chocolate and beat until combined. Add egg and vanilla and beat until incorporated.

Reduce speed to low and gradually add flour mixture, scraping bowl as needed, until a soft, slightly sticky dough forms. Divide dough into two portions, form into 1-inch-thick disks, and wrap in plastic wrap. Refrigerate for at least 30 minutes or until ready to use.

Heat oven to 325°. Working with one dough at a time, roll into ¼-inch thickness on lightly floured work surface. Use a 3-inch Christmas tree cookie cutter and stamp out cookies. Transfer to parchment-lined baking sheets. Bake until set, about 10 minutes. Transfer to wire rack to cool completely before decorating.

To make glaze, whisk together cream cheese and milk until smooth. Add cocoa powder and confectioners’ sugar and whisk until smooth.

Working with one cookie at a time, spread glaze on cookie and immediately sprinkle with ground pistachios. Continue with remaining cookies.