Begin by browning the butter. Have a heatproof bowl ready to transfer the butter into once it’s browned. Set this aside. Place ½ cup plus 2 tablespoons of unsalted butter in a large, light-colored skillet (so you can easily watch the color change) over medium heat. Stir continuously with a rubber spatula to prevent the milk solids from burning. Once melted, the butter will start to foam and bubble around the edges—keep stirring! After a few minutes, the butter will turn a golden brown and develop a foamy layer of bubbles on top. Soon, the foam will begin to dissipate, revealing perfectly toasted brown milk solids at the bottom of the pan, and the butter will emit a strong, nutty aroma. Immediately remove the pan from the heat and pour the browned butter into the prepared heatproof bowl to halt the cooking. Leaving it in the hot pan risks burning the milk solids quickly. Place the bowl in the fridge to cool for 30 minutes.

While the brown butter cools, prepare the Nutella centers. Line a baking sheet with parchment paper, and using two spoons, scoop out 12 equal portions of Nutella—about 1 tablespoon each. Place the tray in the freezer so the Nutella firms up.

Using a hand or stand mixer fitted with the paddle attachment, cream together the cooled browned butter (it should be partially solid and no longer warm) and ¾ cup packed light brown sugar for a couple of minutes, until the mixture lightens in color. Scrape down the sides and bottom of the bowl as needed with a rubber spatula. Add 1 large egg, 1 tablespoon sour cream or Greek yogurt, and 1 teaspoon vanilla extract, mixing on medium-high speed until smooth.

In a separate bowl, whisk together 1¼ cups all-purpose flour, ½ teaspoon baking soda, and ½ teaspoon fine salt. Slowly add the dry ingredients to the wet ingredients, mixing on low speed just until combined. Gently fold in 1 cup of roughly chopped chocolate chips.

The dough will be slightly sticky. Use a cookie scoop to portion 12 equal-sized balls of dough and place them on a parchment-lined baking sheet. Chill the dough balls for at least 30 minutes, or up to overnight. This resting time allows the dough to fully absorb the brown butter flavor, hydrates the flour, and prevents the cookies from spreading too much during baking.

About 20 minutes before baking, preheat the oven to 350°F (175°C). Remove the chilled dough and frozen Nutella scoops from the freezer. Flatten each dough ball in your palm, place a frozen Nutella scoop in the center, and fold the dough around it, shaping it into a puck, making sure to completely enclose the Nutella. Repeat with the remaining dough. If the Nutella softens too much or the dough sticks to your hands, return the cookies to the fridge or freezer until easier to handle.

Line two baking sheets with parchment paper or silicone mats. Place the cookie dough balls 2 to 3 inches apart on the sheets. Optionally, sprinkle extra chocolate bits on top. Bake for 9 to 10 minutes, until the edges just start to brown. Rotate the pans halfway through baking for even cooking. Remove from the oven and immediately sprinkle with flaky sea salt while the cookies are still warm.

Allow the cookies to cool on the baking sheet for 15 minutes before transferring them to a wire rack to cool completely. Enjoy your cookies with a cold glass of milk!