Lightly spoon flour into dry measuring cups; level with a knife. Combine flour and next 5 ingredients (through salt), stirring well with a whisk. Melt 2 tablespoons butter. Combine 2 tablespoons melted butter, milk mixture, rind, and egg in a large bowl, stirring well with a whisk. Add flour mixture, stirring until well blended (dough will be crumbly). Turn dough out onto a lightly floured surface; knead lightly 7 times. Shape dough into a 4 x 12–inch log on a baking sheet coated with cooking spray.
Bake at 350° for 20 minutes or until golden brown. Reduce oven temperature to 325°. Remove log from baking sheet; cool 10 minutes on a wire rack. Using a serrated knife, cut log diagonally into 24 (1/2-inch) slices. Place slices upright on baking sheet; bake at 325° for 20 minutes or until crisp. Remove from baking sheet; cool completely on wire rack.
Combine remaining 1 teaspoon butter and chocolate in a microwave-safe dish; microwave at HIGH 30 seconds or until chocolate melts, stirring once. Pour chocolate mixture into a small zip-top plastic bag; seal. Snip a tiny hole in 1 corner of bag; drizzle chocolate mixture over cooled biscotti. Let stand 30 minutes.