In a medium bowl, stir together flour, cocoa powder, and salt; set aside. In a large mixing bowl, beat butter with an electric mixer on medium speed for 30 seconds. Add powdered sugar and vanilla. Beat until combined, scraping sides of bowl occasionally. Beat in egg. Beat in as much of the flour mixture as you can. Stir in any remaining flour mixture and the nuts. Cover and chill dough about 1 hour or until dough is easy to handle
Preheat oven to 350 degree F. Shape dough into 3/4-inch balls. Roll balls in granulated sugar to coat. Place balls on ungreased cookie sheets. With the bottom of a glass, flatten balls to about 1-1/4-inch circles.
Bake in the preheated oven for 6 to 8 minutes or until tops are just firm. Transfer cookies to a wire rack and let cool.
Spread bottoms of half of the cookies with a rounded 1/2 teaspoon White Chocolate-Hazelnut Filling. Top with remaining cookies, bottom sides down. Makes 48 cookie sandwiches.
White Chocolate-Hazelnut Filling
In a small saucepan, heat and stir white chocolate and whipping cream over low heat until just melted. Remove from heat. Stir in finely chopped toasted hazelnuts.