Preheat your oven to 350°F (175°C) and line a baking sheet with parchment paper.
In a large mixing bowl, cream together the softened butter and granulated sugar until the mixture is light and fluffy, and the sugar begins to dissolve, about 2-3 minutes. You should see a pale, smooth batter with a gentle aroma of sweetness.
Beat in the egg and vanilla extract until fully incorporated, resulting in a glossy, cohesive mixture. The dough will become slightly thicker and smoother.
Gradually add the flour to the wet ingredients, mixing on low speed or stirring by hand, until just combined. The dough will come together and feel soft but manageable; it may be slightly sticky.
Using your hands or a cookie scoop, form the dough into 1-inch balls and place them evenly spaced on the prepared baking sheet. Gently flatten each ball slightly with your palm for a more uniform shape.
Press your thumb or the back of a small spoon into the center of each dough ball to create a deep well around 1/2 inch wide. Be careful not to press too hard to keep the edges intact.
Fill each well with about 1/2 teaspoon of Nutella or hazelnut spread. The filling should sit neatly within the indentation without overflowing.
Bake in the preheated oven for 10-12 minutes, or until the edges are lightly golden and the Nutella filling is slightly bubbly. The cookies will firm up around the edges while remaining soft inside.
Remove the cookies from the oven and let them cool on the baking sheet for 5 minutes, allowing the centers to set slightly.
Transfer the cookies to a wire rack and let them cool completely before serving, so the Nutella sets fully and the cookies develop a delightful texture.
Enjoy these cookies with a glass of milk or tea, marveling at the combination of crispy edges and gooey chocolate-hazelnut center with every bite.
