1. In the bowl of an electric mixer, cream butter and sugar until well blended, about 1 to 2 minutes. Add vanilla, lemon extract, and salt. With mixer on low speed, gradually add flour, scraping down sides of bowl.

2. Turn dough out onto work surface; it will be loose and crumbly. Knead dough by pushing small amounts away from you with the heel of your hand for 1 to 2 minutes. Divide dough in half. Sprinkle cocoa powder over one half. Continue to knead until cocoa is fully incorporated. Using a rolling pin, shape each half of dough between plastic wrap into a 7-by-7-inch square, 1/2-inch thick. With a sharp knife, slice each dough into nine 3/4-inch strips; use a ruler to measure the precise 3/4-inch increments.

3. Whisk together egg and 1 tablespoon water. Cover work surface with plastic wrap. Place three strips of dough on plastic, alternating white and chocolate. Brush tops of strips and in between with egg wash. Press together gently. Form second and third layers by adhering alternate colors with egg wash and stacking to create the effect of a checkerboard. Wrap log in plastic wrap. Repeat process for second log, reversing color pattern. Refrigerate for 30 minutes, or freeze for 15 minutes.

4. Heat oven to 350 degrees. Slice each log into 1/4-inch thick slices, and place on an Exopat-lined baking sheet. Bake for 10 to 12 minutes. Let cool for 2 minutes, and transfer to baking rack. Allow to cool completely.

strips of dough. Similar to French sabl, s, these tender butter cookies have a light, crumbly texture. The dough is lightly kneaded, which gives the cookies a flaky texture. Martha Stewart incorporates Valrhona cocoa powder into half the dough, which gives it a rich flavor and dark color. The cookie dough can be made a day or two in advance, or even frozen up to several weeks.