Prepare the dough: In a large mixing bowl, beat the softened butter and granulated sugar with an electric mixer on medium speed for 2–3 minutes, until light and creamy.
Add wet ingredients: Add the egg yolks and vanilla extract. Beat until smooth and well combined, scraping down the sides of the bowl as needed.
Combine dry ingredients: In a separate bowl, whisk together the all-purpose flour, cornstarch, and salt.
Make the cookie dough: Gradually add the dry ingredients to the butter mixture, mixing on low speed until a soft dough forms. Avoid overmixing.
Chill the dough: Cover the dough and refrigerate for 20 minutes, or until firm enough to roll without sticking.
Preheat the oven: Preheat the oven to 350°F (175°C). Line two large baking sheets with parchment paper.
Scoop approximately 1 tablespoon of dough per cookie and roll into smooth balls. Spread the chocolate sprinkles on a plate, then roll each cookie dough ball in the sprinkles until evenly coated. Arrange the coated dough balls on the prepared baking sheets, spacing them about 2 inches apart.
Make the indentations: Using your thumb or the back of a rounded measuring spoon, gently press an indentation into the center of each cookie. If the edges crack, smooth them with your fingers.
Bake: Bake for 12–14 minutes, or until the edges are lightly golden and the centers are set. If the indentations puff up during baking, gently press them again immediately after removing the cookies from the oven.
Cool: Allow the cookies to cool on the baking sheet for 5 minutes before transferring them to a wire rack to cool completely.
Fill with Nutella: Transfer the Nutella to a piping bag or a small zip-top bag with the corner snipped off. Pipe approximately 1 teaspoon into the center of each cooled cookie. Allow the filling to settle before serving.