Preheat the oven to 350°F. Grease an 8x8-inch square baking dish and line it with parchment paper, leaving an overhang for easy lifting.

Make the cookie layer: in a large bowl, beat 1 stick (1/2 cup) softened salted butter, 1/3 cup dark brown sugar, and 2 tablespoons granulated sugar until combined and slightly fluffy. Add 1 large egg and 1 teaspoon vanilla extract and beat until incorporated.

Add 1 cup + 2 tablespoons all-purpose flour, 1/2 teaspoon baking soda, and 1/2 teaspoon kosher salt to the butter mixture. Beat or stir just until combined. Fold in 3/4 cup semi-sweet chocolate chips.

Press the cookie dough evenly into the prepared pan to form a uniform layer covering the bottom.

Make the Nutella brownie layer: in a medium bowl, whisk together 1 cup Nutella, 2 large eggs, 4 tablespoons melted salted butter, and 2 teaspoons pure vanilla extract until smooth.

Add 1/2 cup all-purpose flour to the Nutella mixture and mix until just combined. Fold in 1/2 cup semi-sweet chocolate chunks, if using.

Spread the Nutella batter evenly over the cookie layer in the pan.

Bake uncovered in the preheated 350°F oven for 30 minutes. After 30 minutes, tent the pan with foil and bake an additional 10–15 minutes, until the center is just set (a slight jiggle is okay).

Remove from the oven and immediately sprinkle flaky sea salt over the top. Cool in the pan on a wire rack until mostly cooled (at least 1 hour). Use the parchment overhang to lift the bars from the pan, then slice.

Serve warm or cooled. Store leftovers in an airtight container for up to 5 days.