1. Preheat your oven to 350°F (175°C) and line a baking sheet with parchment paper.
2. In a bowl, mix together the pistachio flour, almond flour, coconut flour, baking powder, and salt.
3. In another bowl, cream the softened butter and sugar until light and fluffy.
4. Add the egg and vanilla extract to the butter mixture and mix well.
5. Gradually add the dry ingredients to the wet ingredients, mixing until combined.
6. Using a cookie scoop, drop dough onto the prepared baking sheet, spacing them about 2 inches apart.
7. Bake for 10-12 minutes or until the edges are lightly golden.
8. Let the cookies cool completely on a wire rack.
9. Meanwhile, melt the white chocolate chips in a microwave-safe bowl, stirring until smooth.
10. Once the cookies are cool, spread a layer of melted white chocolate on the bottom of one cookie and sandwich it with another cookie.
11. Repeat with the remaining cookies and allow the chocolate to set.

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