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.