Created by Dreamer_in_Ontario on December 7, 2012
Step 1: Add butter and sugar to large bowl and beat with electric mixer until well blended and light in colour (1-2 minutes)
Step 2: Beat in vanilla, rum flavoring (if using) and salt; add flour a bit at a time and mix on low speed until crumbly.
Step 3: Turn dough out onto a lightly floured surface and knead until dough holds together.
Step 4: Divide dough in half and return half of dough to bowl and knead in the cocoa powder.
Step 5: To make spiral shaped cookies, divide chocolate and vanilla dough in half.
Step 6: On well-floured surface, roll each piece of dough into a 6" x 8" rectangle; place 1 rectangle of chocolate dough onto piece of parchment paper; lightly bruch with egg wash.
Step 7: Using a spatula, carefully lift vanilla dough and place on top of chocolate dough, making sure edges are even (trim if necessary), seal the edges and lightly brush again with egg wash.
Step 8: Using parchment paper, roll cookie dough from the long way, jelly roll style, into a tight cylinder; wrap dough in plastic wrap and refrigerate for 30 minutes; repeat with remaining dough.
Step 9: Preheat to 325F; line baking sheets with parchment paper: cut rolls into 1/4" slices and bake for 14 to 16 minutes or until lightly golden; transfer to wire rack to cool.
Step 10: Checkerboard pattern: Cut the vanilla rolled dough into 5 3/8" strips; cut chocolate dough into 4 3/8" strips; brush with egg wash and arrange in 3 layers of 3 strips each in alternating colors; wrap the resulting rectangle in a thin sheet of dough which has been egg washed; continue as directed with spiral pattern.