Preheat the oven to 350°. Lay out 2 pieces of parchment paper on 2 large cookie sheets. Set aside.

Combine the flour, cocoa powder, cornstarch, baking powder, and salt in a medium bowl and whisk.

In a separate large bowl, use an electric hand mixer to mix the butter and sugar until they are fluffy.

Add the egg and vanilla extract, and continue mixing.

Pour in the dry ingredients, and mix at low speed until the dough is crumbly and pliable.

Use a 1 1/2 tablespoon cookie scoop to scoop out balls of dough and roll them with your hands until they are smooth, tight balls. Set them on the cookie sheets about 2 inches apart. (I fit 12 cookies on 1 cookie sheet.)

Press down the center of each cookie slightly with a measuring teaspoon. It’s okay if the dough cracks a little bit. 

Bake the cookies, 1 cookie sheet at a time, for 15 minutes or until they have risen slightly. Press the centers of the cookies down again if they have risen too much while baking.

Rest the cookies on a cooling rack for about 5 minutes (or until they are still warm but not hot to the touch), then press a peppermint kiss into the center of each cookie.

Let the cookies cool completely, then serve.