How to Make It
Preheat oven to 350°F. Spread hazelnuts in a single layer on a rimmed baking sheet, and toast in preheated oven until fragrant and blistered, about 10 minutes. Transfer hot toasted hazelnuts to a clean kitchen towel, and rub together to remove skins. Discard skins. (It’s OK if not all the skins come off.) Let nuts cool completely, about 15 minutes. Coarsely chop cooled nuts using a knife or food processor. Reserve 2 tablespoons chopped toasted hazelnuts.
Line an 8-inch square baking pan with parchment paper, allowing 2 inches of overhang on all sides. Whisk together remaining chopped toasted hazelnuts, graham cracker crumbs, coconut, melted butter, eggs, granulated sugar, cocoa, and vanilla in a large bowl until well combined. Press mixture firmly into prepared baking pan, using palm of hand to make sure it is as flat and even as possible. Bake in preheated oven until just dry to the touch, about 10 minutes. Transfer baking pan to a wire rack, and let cool completely, about 15 minutes.
Beat powdered sugar, butter, cornstarch, milk, vanilla, and salt with a heavy-duty stand mixer fitted with the paddle attachment on high speed until well combined and fluffy, about 3 minutes. Spread mixture evenly over crust in baking pan. Chill at least 15 minutes while you make the ganache.
Combine chocolate and butter in a microwavable bowl, and microwave at high until butter is melted, stirring at 20-second intervals. Stir mixture until smooth and thick. Spread ganache evenly over chilled filling in baking pan. If desired, spoon dollops of tahini on top of ganache, and gently swirl with the tip of a knife, making sure not to disturb the filling underneath. Sprinkle with reserved 2 tablespoons chopped toasted hazelnuts.
Chill until ganache is set, about 1 hour. Using a sharp knife, cut into 24 bite-size bars. Bars can be stored in an airtight container in refrigerator up to 3 days.