- 1 pound unsalted butter, room temperature
- 1 cup sugar
- 2 large eggs
- 1 egg white
- 2 teaspoon vanilla extract
- 1/4 teaspoon salt
- 5 cup flour, sifted
- 3 tablespoon Dutch-process cocoa
Beat the butter using a mixer set on medium-high speed until light and fluffy. Add the sugar gradually and continue to beat until pale and fluffy. Add the whole eggs one at a time and the vanilla. Combine the salt and flour and stir it into the butter mixture. Remove half of the dough and set aside. Mix the cocoa into the remaining dough on low speed, until incorporated. Pat the vanilla and the chocolate doughs each into an 11- by 9-inch rectangle. Wrap each in plastic wrap and chill in the refrigerator until firm.
Brush the vanilla dough with egg white and place the chocolate dough on top. Press gently and cut in half lengthwise. Brush one half with egg white and stack the remaining half on top, making certain the vanilla and chocolate doughs alternate. Cut the resulting stack in half lengthwise and set one stack aside. Slice one stack into thirds lengthwise and flip the middle stack over; lightly brush the sides with egg white and gently press together to form a checkerboard-patterned log. Cover with plastic wrap and chill until firm. Repeat with the remaining stack.
Heat oven to 350. Line baking pans with parchment paper. Slice a checkerboard log crosswise into 1/4-inch cookies. Place the cookies on the baking pans, 1 inch apart, until firm — 7 – 10 minutes. Rotate pans once while baking. Cool completely on a rack.