1. Prepare chocolate dough: In the bowl of a stand mixer cream the butter and sugar until light and fluffy. Add the yolks and beat well. On low speed, sift in the cocoa, flour and salt. Set aside.

2. Repeat the directions for the vanilla dough, adding the vanilla with the butter and sugar.

3. Roll out each flavored dough into a ½-inch thick rectangle.

Note: It is easiest to roll the dough between two sheets of parchment paper.

4. Place the rectangles of dough on cookie sheets and refrigerate until firm, about 1 hour.

5. Preheat the oven to 350 degrees.

6. To assemble checkerboards, cut each rectangle into ¼-inch long slices. Lightly brush each slice with egg white. A light wash is essential—too much and it will be a gooey mess, not enough and the checkerboards won’t stick together.

7. Alternate chocolate and vanilla slices to create a block of dough with a checkerboard effect, pressing to help the slices adhere to each other. Your block should have three rows of three slices each, stacked three high. Press the block firmly together and freeze until firm, about 15 minutes.

8. Slice the checkerboard stack into ¼ inch slices and arrange slices on baking sheets, then place the baking sheets in the freezer for another 10 minutes.

9. Bake in the preheated oven for 10 minutes.

10. Remove from the baking sheet and cool on a metal rack.