Preheat the oven to 350°F (175°C).

In a large mixing bowl, cream together the softened butter and granulated sugar until light and fluffy.

Add the egg and vanilla extract to the creamed mixture, combining until smooth.

In a separate bowl, whisk together the flour, salt, and cocoa powder.

Gradually add the dry ingredients to the wet mixture, mixing until just combined.

Divide the dough in half. Add cocoa powder to one half and food coloring to the other if desired. Mix until evenly colored.

Roll out each half of the dough separately between sheets of parchment paper into rectangles about 1/4 inch thick.

Place one dough layer on top of the other, press gently, and then roll them up tightly in the parchment paper.

Wrap the rolled dough in plastic wrap and refrigerate for at least 30 minutes or until firm.

Slice the chilled dough into 1/4 inch thick pieces and place on a baking sheet lined with parchment paper.

Bake in the preheated oven for 10–12 minutes, or until the edges start to get a golden color.

Allow the cookies to cool on the sheet for a few minutes before transferring them to a wire rack.