These cookies are called chocolate diamonds because they are rolled in sugar, which makes them sparkle. It is very important to use cake flour in this recipe because its low protein (gluten) level will make the cookies melt in your mouth.
Using a stand mixer, combine the butter, sugar and 1/3 of the vanilla bean's seeds. Divide the mixture in half and remove one half from the mixing bowl. To the remaining half, add the cocoa powder, scant 3 cups of the cake flour and another 1/3 of the vanilla bean's seeds while mixer is on. Mix and remove from the bowl. Place the other half back in the mixing bowl and add the remaining 3 cups cake flour, the remaining 1/3 vanilla bean's seeds and the cinnamon. Place both doughs in the refrigerator for 1 hour.
Remove the 2 doughs from the refrigerator. It may be necessary to beat the dough with a rolling pin to soften the butter and make the dough more pliable. Roll the chocolate dough into a rope about 3/4-inch in diameter. Roll the white dough about 2 inches in diameter and the same length as the chocolate rope. Trim one side of the dough to create a straight edge. Using a pastry brush, spread a little water on top of the white dough. The moisture will help the dough stick together. Place the chocolate rope on top and roll the white dough around it. Trim a straight edge where the doughs meet. Close the seam with a little more water. If the dough is too soft, place in the refrigerator or freezer until firm.
Barely wet the surface of the rolled dough and roll it in granulated sugar. Using a sharp chef's knife, make 1/2-inch slices. Place the slices on a parchment-lined sheet pan and bake at 375 degrees F for 15 minutes.