In a large bowl or stand mixer beat butter and sugar until creamy then add the egg and vanilla and beat to combine well. Divide the mixture evenly into two evenly parts.
Starting with the vanilla dough, place half the mixture in the stand mixer bowl (or large bowl) sift in the flour, salt and corn starch combine on low until it has almost combined into a dough, move the mixture to a lightly floured flat surface and gently knead to form a soft compact dough, do not over knead.
In the same bowl add the remaining butter mixture, sift in the flour, salt, corn starch and cocoa, combine on low until it has almost combined into a dough, move the mixture to a lightly floured flat surface and gently knead to form a soft compact dough, do not over knead.
Wrap each dough ball with plastic wrap and chill 30 minutes.
Roll out the vanilla dough to a thickness of a little less than ½ inch / 1 cm, approximately 10.5 inches/ 26 cm long and cut out 4 strips a little more than ½ inch / 1.5 cm wide as you see them in the photo. Roll out the cocoa dough to a thickness of a little less than ¼ inch / 5 mm and cut out 2 strips a little less than ¼ inch / .6 cm wide, approximately 10 inches/ 26 cm long.
From the cocoa dough, cut another piece, still to the thickness of a little less than ¼ inch / 5 mm thick but 1.2 inches / 3.5 cm wide, approximately 12 inches/ 30 cm long, this strip should be the same size as the combination of 2 vanilla and 1 cocoa strip together.
Lightly brush the egg white along the inside edges of the vanilla dough, place a cocoa dough strip in the middle of two vanilla strips, repeat with the remaining 3 strips. Brush the top of one of the completed strips lightly with egg white then place the large cocoa strip on top, again lightly brush with egg white and place the remaining completed strips on top.
Wrap the dough with plastic wrap and chill for 1 hour.
Cut the dough with a sharp knife into approximately ¼ inch / 1.2 cm slices, place the slices cut side up on parchment paper lined cookie sheets, make small triangles and balls with the remaining dough and decorate, brush the top of the cookies again with egg white if necessary for the decorations to stick. Refrigerate for 30 minutes before baking.
Pre-heat oven to 350F/180C.
Bake for approximately 13-15 minutes, let cool 10-15 minutes on the baking sheet then move to a wire rack to cool completely. Enjoy!
Notes
Be sure to chill the dough, the cookie dough must be cold.I made pinwheel cookies with the remaining dough. Roll each dough to ⅛inch thickness, place the cocoa dough on top of the vanilla and roll up tightly, brush the dough with a lightly with milk to help seal the edges. Wrap in plastic and refrigerate for one hour. Cut the roll into ¼ inch slices and place on a parchment paper lined cookie sheet. Bake for 10 minutes. Let cool 10 minutes on the baking sheet then move to a wire rack to cool completely.