In the bowl of a stand mixer fitted with the paddle attachment, cream together 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, baking soda, baking powder, salt, all-purpose flour and dark cocoa powder. Mix just until a soft chocolate cookie dough forms. Be careful not to over mix.
Using a medium cookie scoop, portion the dough into 20 even cookie dough balls. Roll each dough ball in year hands to gently warm up the dough for 10 seconds then press into the brown nonpareils, gently pressing the sprinkles into the dough so they adhere.
Place the 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, spacing them approximately 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 the back of a tablespoon measuring spoon to gently press an indentation into the center of each cookie. If desired, use a round cookie cutter to gently shape the cookies before making the indentation. Allow the cookies to cool on the baking sheet for 5 minutes before transferring them to a wire rack to cool completely.