1.  To prepare cookies, preheat oven to 350F. Line large baking sheet with parchment paper. 

2.  In medium bowl, whisk together flour, pistachio meal, and salt. Set aside. In bowl of stand mixer fitted with paddle attachment, beat together butter and sugar on medium speed until creamy, about 30 seconds. Beat in egg yolk and vanilla and mix until well combined. Reduce speed to low and add flour mixture, mixing until ingredients are incorporated and doug comes together in a ball, about 45 seconds. Shape dough into a disk, tightly wrap in plasic wrap, and chill in refrigerator for 30 to 60 minutes. 

3.  Lightly flour work surface. With rolling pin, roll out chilled dough until 1/8 inch thick. Using your favorite cookie cutter, cut out cookies. For every cookie you cut, you will need to cut out an identical cookie, which will be the top cookie in the sandwich and have a cutout in the center. Place cookies on prepared baking sheet and use a smaller cookie cutter to cut out centers of half the cookies. Bake 6 - 8 minutes, or until bottoms are golden. Cool cookies on baking sheet 10 minutes before transferring to wire rack and allowing to cool completely. 

4.  To prepare ganache, place white chocolate in medium heatproof bowl. In small saucepan over medium heat, heat cream 2 - 3 minutes, until it starts to lightly simmer. Remove cream from heat and pour on top of white chocolate. Let sit 2 - 3 minutes. Whisk until chocolate has fully melted. Whisk in lavender. Allow to cool at least 30 minutes before using. If still runny, place in refrigerator for 15 minutes. 

5.  Dust all cutout cookies with powdered sugar. With a spoon, fill bottom cookies with large dollop of ganache and place cutout cookies on ganache immediately.