Preheat oven to 300 degrees, with rack in center. Lightly butter a 10-inch springform pan, and set aside.
Stir together flour, salt, and 1/2 teaspoon of each ground peppercorn; set aside. Put butter and confectioners' sugar into the bowl of an electric mixer fitted with the paddle attachment, and mix on medium speed until fluffy, about 2 minutes, scraping down sides of bowl as needed. Reduce speed to low. Gradually add flour mixture, and mix until combined.
Using a small offset spatula, spread dough evenly into prepared pan, smoothing top. Bake until shortbread is golden brown, about 1 hour 20 minutes.
Transfer pan to a wire rack. Immediately sprinkle with granulated sugar and reserved 1/4 teaspoon of each ground peppercorn. Remove springform; let shortbread cool completely. Cut into wedges.
This was my first attempt at shortbread - it turned out great, melt in your mouth buttery. The only thing I'd suggest is to take it easy on the white and pink pepper corns, they have a slightly sour flavor that works nicely but I'd like it to be a little less prominent.
OH. MY. HECK. Try this with a basic royal icing as a coup de grn n n n ce but add Fleur de Sel or hand crushed Pink Himalayan Salt as a finishing touch while the icing is wet. Absolute bliss.