Position a rack in the center of the oven and heat to 350°F. Line an 8x8-inch metal baking pan with parchment paper, leaving a 2-inch overhang on two sides for easy lifting.

Combine butter and chocolate in a medium saucepan over medium-low heat. Stir constantly until melted and smooth, about 4 minutes. Remove from heat and let cool 5 minutes until warm to the touch, not hot.

Whisk sugar into the warm chocolate mixture until glossy and well combined. Add eggs one at a time, whisking vigorously after each until fully incorporated. Stir in vanilla.

Switch to a rubber spatula. Add flour, cocoa powder, and salt, folding just until no streaks of flour remain. Do not overmix - the batter will be thick and glossy.

Spread half the batter in the prepared pan. Dollop 3 tablespoons of warmed Nutella in spaced spoonfuls over the surface. Top with remaining batter, spreading gently to cover. Dollop remaining 5 tablespoons Nutella on top. Use a butter knife to swirl through the batter in figure-eight motions, creating marbled ribbons without fully blending.

Bake 32-36 minutes, until the top is crackly and a toothpick inserted in the center comes out with moist crumbs attached but no wet batter. The center should feel set but still soft. Cool completely in the pan on a wire rack, at least 2 hours, before lifting out and cutting into 16 squares.