Gâteau Creusois (Creuse Hazelnut Cake)

Instructions

1. Heat the oven to 350°F (175°C). Spread all the hazelnuts (for the cake and the topping) on a baking sheet and toast 8 to 10 minutes, until fragrant and the skins crack. Rub them in a clean towel to remove most of the skins and let cool, about 1 hour.
2. Pulse 125 g of the hazelnuts with 2 tablespoons of the all-purpose flour in a food processor in short bursts until finely ground, stopping before they turn oily. Coarsely chop the remaining 25 g of hazelnuts for the topping.
3. Melt the unsalted butter in a small saucepan over medium heat and cook, swirling, until it foams and the solids at the bottom turn light brown and smell nutty, 4 to 5 minutes. Pour into a bowl and let cool until just warm, about 30 minutes. Butter the cake pan and line the bottom with parchment.
4. In a large bowl, whisk the egg whites with the fine salt until frothy, about 1 minute; they should not form peaks. Whisk in the granulated sugar for about 1 minute, until foamy and glossy.
5. Fold in the ground hazelnuts and the rest of the all-purpose flour, then stir in the vanilla extract and the warm browned butter, scraping in the browned bits, until smooth.
6. Pour the batter into the pan and scatter the chopped hazelnuts over the top. Bake 30 to 35 minutes, until golden with a thin crackly top, the center springs back when pressed and a skewer comes out with a few moist crumbs.
7. Cool in the pan for 15 minutes, then run a knife around the edge, turn the cake out and set it right side up on a rack to cool completely, about 1 hour.