Line a small baking sheet or plate with parchment paper. Using a teaspoon or small cookie scoop, drop 16–18 mounds of Nutella (about 1 teaspoon each) onto the parchment. Place in the freezer for at least 30 minutes, or until firm. This helps keep the centers gooey and makes them easier to wrap in dough.

In a medium bowl, whisk together the all-purpose flour, baking soda, baking powder, and salt. Set aside.

In a large bowl, beat the softened butter, brown sugar, and granulated sugar with a hand mixer (or stand mixer with paddle attachment) on medium speed for 2–3 minutes, until light, fluffy, and slightly paler in color.

Beat in the egg, egg yolk, and vanilla extract until fully combined and glossy, scraping down the sides of the bowl as needed.

Add the milk and mix just until incorporated to help create a moist, chewy cookie texture.

Add the dry ingredient mixture to the wet ingredients in two additions, mixing on low speed just until the flour is incorporated. Do not overmix; a few faint streaks of flour are fine.

Using a spatula, gently fold in the mini chocolate chips, if using. The dough should be soft but not overly sticky. If it feels very soft or your kitchen is warm, cover and chill the dough for 20–30 minutes.

Preheat your oven to 350°F (175°C). Line two large baking sheets with parchment paper or silicone baking mats.

Scoop about 2 tablespoons of dough per cookie (a medium cookie scoop works well). Flatten each portion slightly into a disc in the palm of your hand.

Remove the frozen Nutella mounds from the freezer. Place one frozen Nutella piece in the center of each dough disc. Gently wrap the dough around the Nutella, pinching to seal, then roll lightly between your hands to form a smooth ball. Make sure no Nutella is exposed.

Place the stuffed dough balls on the prepared baking sheets, spacing them about 2 inches apart. Bake for 10–12 minutes, until the edges are set and just barely golden while the centers still look soft and slightly underbaked.

Let the cookies cool on the baking sheet for 5–10 minutes before transferring to a wire rack. They will continue to set as they cool while the Nutella center remains thick and gooey. If desired, sprinkle with flaky sea salt while warm and drizzle with a little warmed Nutella before serving.