Whisk together flour, corn starch, cocoa powder, and salt.
2 cups flour, 2 Tbsp corn starch, ¾ cup cocoa powder, ½ tsp salt
With an electric mixer combine butter and sugar until light and fluffy. Add in egg and vanilla and continue to mix until well blended.
1 cup sugar, 1 cup butter, 1 egg, 2 tsp Vanilla
Gradually add in flour mixture, mixing well after each addition.
Roll dough 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
Carefully slice the 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.