Preheat your oven to 350°F (175°C) and line a baking sheet with parchment paper.
In a large mixing bowl, combine the melted butter, brown sugar, and granulated sugar. Beat them together using an electric mixer until the mixture is smooth and creamy. Add in the egg and vanilla extract, mixing until well incorporated.
In a separate bowl, whisk together the all-purpose flour, baking soda, and kosher salt. Gradually add the dry ingredients to the wet mixture, mixing on low speed until just combined.
Gently fold in the finely chopped dark chocolate or chocolate chips until evenly distributed throughout the cookie dough.
For the best results, cover the dough and chill it in the refrigerator for about 30 minutes.
Once chilled, scoop out a portion of dough and flatten it in your palm. Place a teaspoon of Nutella in the center, then fold the dough over the filling and roll it into a ball. Repeat with the remaining dough.
Place the stuffed cookie balls on the prepared baking sheet, spacing them about 2 inches apart. Bake for 10-12 minutes or until the edges are lightly golden. The centers may look slightly underbaked—that’s perfect for a gooey center!
Allow the cookies to cool on the baking sheet for about 5 minutes before transferring them to a wire rack to cool completely. Enjoy your warm Nutella Stuffed Cookies!