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Make the hazelnut butter - Preheat the oven to 350 degrees F. Place the hazelnuts on a rimmed baking sheet, and bake for 12-15 minutes, until lightly golden brown. Place the warm hazelnuts in a clean towel, and make a bundle, enclosing the hazelnuts. Shake vigorously for 1 minute to remove all of the papery skins from the hazelnuts. Place the hazelnuts in a high powered blender, like the Kitchen Aid K400 Steel Blue Blender, along with the avocado oil, maple syrup, and kosher salt. Blend on medium speed until the mixture reaches a smooth consistency, about 3-4 minutes. Stop the mixture occasionally to scrape down the sides. Scoop the hazelnut butter out into a jar, and set aside.
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Prepare an 8x8" cake pan, by lining it with two thick strips of parchment paper. Fill a small saucepan with 2 inches of water, and bring the water to a simmer. Place a heat proof bowl over the top of the pan. Add the chocolate chips, chopped chocolate, and butter to the bowl. Whisk occasionally until the butter and chocolate have completely melted. Whisk in the pure vanilla extract and instant coffee. Remove the bowl from the heat, and allow to cool slightly.
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In a small bowl, combine the flour, cocoa powder, cornstarch and salt. Whisk to combine and remove any lumps. Set aside.
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In the bowl of an electric stand mixer fitted with the whisk attachment (I love my Kitchen Aid Artisan Steel Blue Stand Mixer) add the eggs, brown sugar, and granulated sugar. Beat on medium speed for 4-5 minutes until the mixture is pale yellow, and airy. With the mixer on low, add the chocolate/butter mixture to the beaten eggs. Whisk to combine. Add the dry ingredients, and whisk on low to just combine. Remove the bowl from the mixer, and use a rubber spatula to continue to gently fold in the dry ingredients until no dry patches of flour remain, making sure to get to the bottom of the bowl.
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Pour the brownie batter into the prepared baking pan. Use a small offset spatula to spread the batter evenly out to the corners of the pan. Drop about 7 tablespoon of the homemade hazelnut butter on top of the brownie batter. Use a wooden skewer or sharp knife to swirl the hazelnut butter into the butter, creating a marble pattern. Bake at 350 degrees for 25-30 minutes, until a cake tester comes out with lots of moist crumbs. Don't overbake! Sprinkle flakey sea salt on top, and allow to cool completely before slicing.