Preheat oven to 350°F. Line two baking sheets with parchment paper. Place the chocolate hearts in the freezer for about 20-30 minutes while you proceed with the cookies.
Using a hand mixer or stand mixer fitted with the paddle attachment, beat butter and sugars for several minutes, until light and creamy. Add the the egg and vanilla extract and beat to combine, scraping the sides of the bowl as necessary.
In a separate bowl, combine the cocoa, flour, baking soda, salt, and cornstarch. With the mixer on low speed, slowly add the flour mixture to the butter mixture, mixing until combined.
Pour the sprinkles onto a large plate (I start with ¼ cup). Scoop the dough into 1½ tablespoon portions (I use a cookie scoop). Gently roll the sides and top of the cookie dough balls into the sprinkles (no need to get sprinkles on the bottom of the cookies). Place the dough balls a couple inches apart on the prepared cookie sheets**. Make sure the dough balls are mounded higher than wider, and not flatted at all (this is so that they don't spread too much).
Bake for 9-11 minutes, until the edges are just beginning to set and the centers are still slightly underdone. They will look under baked, but the cookies will set as they cool.
Place the baking sheets on wire racks. Allow to cool for 2 minutes then place a chocolate heart in the center of each cookie. No need to press it down, the bottom of the heart will start to melt and after several minutes you'll be able to nestle it onto the top of the cookies to help it stick. Allow the cookies to cool completely on the pan on the wire racks.