Step 1

Preparation Line large baking sheet with double thickness of foil. Sift flour, cocoa, baking powder, and salt into medium bowl. Using electric mixer, beat sugar and butter in large bowl to blend. Beat in eggs 1 at a time, then vanilla. Beat in flour mixture. Stir in semisweet and white chips. Drop dough by heaping tablespoonfuls onto prepared sheet in two 10- to 11-inch-long strips, spacing 3 inches apart. Using metal spatula or wet fingertips, shape strips into 11- by 2 1/2-inch logs. Refrigerate 30 minutes.

Step 2

Preheat oven to 350°F. Bake logs until tops are cracked and dry and tester inserted into center comes out clean, about 25 minutes; cool 10 minutes.

Step 3

Reduce oven temperature to 300°F. Using foil as aid, lift logs onto work surface. Line baking sheets with clean foil. Using serrated knife, gently cut warm logs crosswise into 3/4-inch-thick slices. Arrange half of slices, cut side down, on each prepared baking sheet. Bake biscotti until just dry to touch, about 8 minutes. Turn biscotti over. Bake until top is dry to touch, about 8 minutes. Cool on sheets.