Line a small baking sheet or plate with parchment paper. Using a small spoon or mini cookie scoop, drop 18–20 teaspoon-sized dollops of Nutella onto the parchment. Place the tray in the freezer for at least 30–45 minutes, or until the Nutella mounds are firm enough to handle.

In a large mixing bowl or the bowl of a stand mixer, add the softened butter, granulated sugar, and brown sugar. Beat with a hand mixer or stand mixer on medium speed for 2–3 minutes, until the mixture looks light, fluffy, and slightly paler in color.

Add the egg and vanilla extract to the creamed butter-sugar mixture. Beat on medium speed for 30–45 seconds, just until well combined and smooth. Scrape down the sides and bottom of the bowl so there are no streaks of egg.

In a separate medium bowl, whisk together the flour, cornstarch, baking soda, and salt until evenly combined and free of clumps.

Add the dry ingredient mixture to the wet ingredients in two batches. Mix on low speed or stir with a spatula just until the flour disappears and a thick, slightly sticky dough forms. If using chocolate chips, gently fold them in now.

Cover the bowl of dough and refrigerate for 30–45 minutes, or up to 24 hours. If chilling longer than 2 hours, let the dough sit at room temperature for 10–15 minutes before shaping so it is easier to scoop.

When ready to bake, preheat the oven to 350°F (175°C). Line two baking sheets with parchment paper or silicone baking mats.

Use a medium cookie scoop (about 1 1/2 tablespoons) or a spoon to portion the chilled cookie dough into 18–20 equal balls. Flatten each ball slightly into a disk in the palm of your hand. Place one frozen Nutella mound in the center of each disk, then fold the dough up and around the Nutella, pinching to fully seal the edges. Roll gently into a smooth ball and place on the prepared baking sheets, spacing about 2 inches apart.

Bake one sheet at a time on the middle rack for 10–12 minutes, or until the edges look just set and lightly golden and the tops no longer look shiny but still appear soft. The centers should still feel slightly soft if gently tapped.

Let the cookies cool on the baking sheet for 10 minutes—they will be fragile straight from the oven. Transfer to a wire rack to cool further, or serve warm within 20–30 minutes for a gooey, molten Nutella center.