Start by melting salted butter in a sauté pan over medium-high heat. Stir constantly for about 5-7 minutes until the butter turns a rich caramel color and emits a nutty aroma.

Preheat your oven to 350°F (175°C) and grease a 9x9 inch baking pan, then line it with parchment paper.

In a large mixing bowl, whisk together cooled brown butter and brown sugar until light and fluffy.

Add eggs one at a time, mixing well after each addition, followed by vanilla extract.

Stir in old-fashioned oats until combined, then sift in flour, baking powder, and cinnamon, mixing just until incorporated.

Scoop Nutella into a microwave-safe bowl and heat for about 15 seconds until pourable.

Spread oatmeal cookie dough evenly in the prepared baking pan, drizzle warmed Nutella over the dough, and swirl it with a knife.

Bake for 25 minutes until golden around the edges. A toothpick should come out with moist crumbs.

Let cool for about 10 minutes in the pan, then lift out using parchment overhang and transfer to a cooling rack.

Sprinkle flaky sea salt on top before cutting into squares.