1/2 cup butter
3/4 cup sugar
1 1/2 cups flours
1/4 cocoa powder
1 egg
1/4 tsp salt

Place all ingredients except the cocoa powder in a food processor. Pulse until it all comes together. Take out of processor and divide dough in two equal parts. Put one part back in the processor and add the cocoa powder. Pulse until combined. 

Roll out both doughs into squares, cover in cling film and refrigerate for one hour.

Take out of fridge and roll out both doughs into rectangles about 1/4 inch thick. Place one on top of the other and start rolling tightly. Cover roll in cling film and refrigerate for one hour again. 

Pre-heat oven to 375. Take out of fridge and cut roll into 1/4 inch thick cookies and place in cookie sheet. Bake for 15 minutes.