Melt butter over medium-high heat, stirring. Cook for about 5-7 minutes, stirring occasionally, until butter solids turn a rich caramel brown color and smells nutty. Immediately transfer to a heatproof container; set aside.
Preheat oven to 350°F. Spray a 9x9 square baking pan with baking spray and line with parchment paper, leaving an overhang on the sides.
Whisk together brown butter and brown sugar in a large bowl until lightened and fluffy.
Whisk in eggs until fully incorporated, followed by vanilla.
Stri in oats with a rubber spatula until well combined.
Sift in flour, baking powder, and cinnamon. Fold with rubber spatula until flour mixture is just incorporated.
Spread dough into prepared pan and smooth with spatula or floured hands.
Heat Nutella in a microwave-safe container about 15 seconds. It should have a thinner, pourable consistency.
Pour nutella over dough and use a butter knife to swirl.
Bake in preheated oven for 25 minutes, or until a toothpick inserted in the center comes out clean or with moist crumbs.
Cool 10 minutes in pan, then transfer to a cooling rack to cool completely.
Sprinkle sea salt on top and cut into bars.
Brown Butter: For a full step-by-step on how to brown butter with images and tips, check out my Brown Butter Appreciation Post.Storage/Freezing: These oatmeal cookie bars freeze well in an airtight container for up to two months, or they can be stored at room temperature for 2 days or in the fridge for up to 4 days.