Preheat oven to 325°F. Line an 8x8x2" glass baking dish with foil, pressing firmly into pan and leaving a 2" overhang. Coat foil with nonstick spray; set baking dish aside.

This is a one bowl recipe, so find a medium sized metal bowl that will fit in a medium sized pan.

Bring an inch of water to a bare simmer in a saucepan. Combine chocolate and butter in bowl and rest in the pan without touching the water. Cook, stirring occasionally, until melted and smooth, about 5 minutes. Remove bowl from heat. Add sugar, vanilla and sea salt and stir until combined. Let cool about 5 to 7 minutes.

Whisk eggs into chocolate mixture one at a time. Add flour and whisk for 30 seconds to thicken the batter. Stir in hazelnuts. Scrape batter into the prepared pan and spread evenly.

Bake until a toothpick inserted into the center comes out clean, about 30 to 35 minutes.

Transfer pan to a wire rack; let cool completely in pan (about 2 hours).

Using foil overhang, lift brownie out of pan; transfer to a cutting board and cut into 16 squares.