In a sauté pan over medium-high heat, melt the salted butter, stirring frequently until golden brown, about 5-7 minutes.

Preheat your oven to 350°F (175°C) and prepare a 9x9 square baking pan.

In a large mixing bowl, whisk the cooled brown butter and brown sugar until light and fluffy. Add eggs one at a time, then stir in the vanilla extract.

Stir in the old-fashioned oats, sift together flour, baking powder, and cinnamon, then fold this into wet ingredients.

Spread the oatmeal cookie dough evenly in the baking pan. Microwave Nutella for 15 seconds until pourable, drizzle it, and gently swirl.

Bake for about 25 minutes until a toothpick inserted comes out clean or with a few moist crumbs.

Let the bars cool in the pan for about 10 minutes, sprinkle with flaky sea salt, and cool completely on a wire rack.