Using a medium bowl, sift the flour and cornstarch together until well combined; set mixture aside.

Using an electric mixer, cream together the butter and sugar until smooth (~3-4 minutes on medium speed).

Add egg yolks and vanilla; mix until well combined.

Add flour mixture to the mixer bowl in 2 additions, mixing until just combined after each addition.

Divide dough into two equal portions. Wrap each portion in plastic wrap and refrigerate for at least 1 hour.

Using a medium bowl, sift the flour, cocoa powder and corn starch together until well combined; set mixture aside.

Using an electric mixer, cream together the butter and sugar until smooth (~3-4 minutes on medium speed).

Add egg yolks and vanilla; mix until well combined.

Add flour mixture to the mixer bowl in 2 additions, mixing until just combined after each addition.

Divide dough into two equal portions. Wrap each portion in plastic wrap and refrigerate for at least 1 hour.

Line 2 baking pans with parchment papers; set pans aside.

Working on a lightly-floured surface, roll/press one piece of vanilla dough into a 4” square; set aside.

Roll one piece of chocolate dough into a 4” square.

Brush top of the vanilla square lightly with water. Lay chocolate square on top and press together lightly. Continue process with the remaining piece of vanilla dough and chocolate dough. Wrap layered dough in plastic wrap and freeze for 30 minutes.

If necessary, trim edges of the dough to keep the square 4”x4”.

Slice the square into (16) ¼” pieces. Lay one piece on its side and brush top with water. Lay another piece on top, rotating the second piece 180° to create a basketweave pattern. Continue process, brushing water in between each layer, until all pieces of dough have been used. Wrap dough in plastic wrap and freeze for 30 minutes.

Preheat oven to 350°F.

Trim off end of log of dough. Slice log into ¼” slices, placing each slice on the prepared baking pans.

Bake for 15-18 minutes, or until cookies begin to turn light golden brown on the edges.

Remove pans from oven and let cookies cool completely before removing.