In a medium saucepan, melt salted butter over medium-high heat, stirring frequently until golden brown (about 5-7 minutes). Transfer to a bowl and let cool.

Preheat your oven to 350°F (175°C) and prepare a 9x9-inch baking pan with non-stick spray and parchment paper.

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

Fold in old-fashioned oats and sifted dry ingredients (flour, baking powder, cinnamon) into the wet mixture until just combined.

Spread the cookie dough evenly into the prepared baking pan. Microwave Nutella for 15 seconds, drizzle it over the dough, and swirl with a knife.

Bake in the preheated oven for approximately 25 minutes. Check for doneness with a toothpick.

Let the bars cool in the pan for 10 minutes. Lift out using parchment overhang, sprinkle with flaky sea salt, and cool completely before cutting into squares.