Pat the shrimp dry with paper towels and place in a large bowl. In a spice grinder, combine the Sichuan and white peppercorns, then grind until medium-fine (about the coarseness of kosher salt). Transfer to a small bowl and stir in the salt. In another small bowl, stir together 2 teaspoons of the pepper mixture, the cornstarch and sugar, then sprinkle over the shrimp and toss. Set remaining pepper mixture aside.