In the bowl of a stand mixer fitted with the paddle attachment, cream the butter and granulated sugar on medium speed for 2 minutes, until light and fluffy. Add the vanilla extract and egg and mix until combined.
With the mixer on low, add the cornstarch, cocoa powder, baking soda, baking powder, kosher salt, and flour. Mix just until a soft chocolate cookie dough forms. Do not over mix.
Using a medium, approximately 2-tablespoon cookie scoop, portion the dough into 20 even balls. Gently roll each ball between your hands for about 10 seconds, then roll in the nonpareils, pressing lightly so they adhere. Place the coated dough balls on a parchment-lined baking sheet, cover, and refrigerate for at least 30 minutes.
Preheat the oven to 350°F. Arrange the chilled dough balls on parchment-lined baking sheets about 2 inches apart. Bake for approximately 10 minutes, until the edges are set but the centers are still soft.
Immediately after removing the cookies from the oven, use a round cookie cutter to gently reshape them, if desired. Use the back of a tablespoon measuring spoon to gently press an indentation into the center of each cookie. Cool on the baking sheet for 5 minutes before transferring to a wire rack to cool completely.