Line 2 baking trays with parchment paper or other non-stick baking surface. Set aside.

In a large mixing bowl with a hand mixer, or in a stand mixer with the paddle attachment, mix together butter, cream cheese, and both sugars. Mix until light and fluffy, about 5 minutes.

Add egg and vanilla, mixing slowly until fully incorporated.

In a separate medium sized bowl, sift together the flour, cornstarch, cocoa powder, baking powder, cream of tartar, and salt.

Now slowly add the flour mixture to the wet ingredients. Mix on low-medium speed until dough forms.

Divide dough in half to make it easier to work with. Roll dough out to about ¼ inch thickness between two sheets of parchment paper. Use a cookie cutter to press straight down into the dough without twisting the cookie cutter. Gently transfer cookies to the baking sheet.

Place the cookies in the fridge to chill for 20 minutes while the oven preheats.

Preheat the oven to 350F.

Bake the cookies for 10-12 minutes or until the edges of the cookie slightly pull away from the paper. Let cool completely before frosting.