Sift the flour, corn starch, cocoa and salt into a large bowl then whisk together to combine.
Combine butter and sugar in a bowl. Mix on medium-low speed with a paddle attachment. Mix until incorporated. Don’t over mix.
Add egg and vanilla then mix and scrape the sides of the bowl.
Add in flour mixture to the bowl. Mix at low speed for 30 seconds.
The dough is ready when it sticks to the paddle mixture. Do not over mix.
Roll out the dough between 2 pieces of parchment paper.
Put on a baking sheet and refrigerate for 1 hour.
Preheat oven to 350 degrees.
Use a cookie cutter to cut out shapes.
Bake for 8-12 minutes or until centers are set.
Allow to cool completely.
Video
Notes
Measure your flour correctly! Adding too much flour to the recipe is the most common mistake. The best, and easiest way to measure flour is by using a scale. If you don't have one then fluff your flour with a spoon, sprinkle it into your measuring cup, and use a knife to level it off.
Make sure to scrape the bowl down several times when making the batter. Stray pieces of unmixed butter of flour will disturb the texture of your cookies.
This is a great dough to make in advance and freeze for on demand cookies later. I like to freeze the dough after rolling out so if defrosts quickly. Make sure to wrap very well before freezing.
I rolled these cookies to 1/4 inch but they could be thicker or thinner. Adjust baking times accordingly.
If you only have salted butter then just use that and reduce the salt added to 1/4 tsp. Unless you love salty chocolate baked goods like I do, then add the full 3/4 tsp.