Whisk together flour, corn starch, cocoa powder, and salt. 

With an electric mixer combine butter and sugar until light and fluffy. Then add in the egg and vanilla and continue to mix until well blended. 

Add in flour mixture a little at a time, mixing well after each addition. 

Take the dough and roll it into a log with a diameter of about 1 ½ inches.

Wrap with parchment or wax paper and place in the refrigerator for 1 hour. 

When ready to bake - Preheat the oven to 350°F

Carefully slice the refrigerated dough into ¼- ½ inch slices and place on a baking sheet. 

Bake for 8-12 minutes or until set. 

Remove from oven and allow to cool on the baking sheet for 2-3 minutes, remove to cooling rack to cool completely.