Hazırlanışı
1Preheat oven to 350°F. Toast hazelnuts on a large rimmed baking sheet until skins are slightly darkened and split, 12–15 minutes. Let cool, then bundle in a kitchen towel and rub vigorously to remove skins (there will definitely be some stubborn pieces that refuse to come off—do what you can). Transfer to a food processor and pulse until finely ground. Whisk hazelnuts, powdered sugar, buckwheat flour, and salt in a medium bowl to combine.
2Cook butter in a small saucepan over medium heat until it foams, then browns (be careful not to let it burn), 5–7 minutes. Let cool slightly.
3Whisk egg whites in a medium bowl until foamy (you’re not trying to create volume; you just want to break them up), then whisk into dry ingredients. Whisk in brown butter. Cover and chill batter until firm, at least 2 hours.