Preheat oven to 350°F and lightly grease an 8-inch skillet.
Brown the butter in a saucepan over medium heat, stirring often, until golden and nutty, about 5–7 minutes. Pour into a bowl and let cool for 15–20 minutes.
Line a small plate with parchment and scoop 10-12 teaspoons of Nutella onto it. Freeze while you prepare the dough.
In a large bowl, whisk together browned butter, brown sugar, and granulated sugar until smooth.
Add egg, vanilla extract, and 3 tablespoons Nutella. Whisk until combined.
Stir in flour, cornstarch, baking soda, salt, and cinnamon until just combined.
Fold in the chocolate chips.
Chill dough in the refrigerator for 20–30 minutes.
Press ⅔ of the dough into the prepared skillet.
Scatter frozen Nutella dollops over the dough.
Top with remaining dough and gently spread to mostly cover the filling.
Sprinkle extra chocolate on top if desired.
Bake for 22–28 minutes, until edges are lightly golden and center is still soft.
Cool for 10 minutes before serving.
Top with flaky sea salt, extra Nutella drizzle, or vanilla ice cream and enjoy