Source: adapted from Suzanne Goin "Sunday Suppers at Lucques" by Brett (who owns the blog site "in praise of sardines")
5 oz. hazelnuts, blanched to remove dark skins (a little over a cup)
½ lb. unsalted butter (plus extra for greasing the pan)
½ vanilla bean
1⅓ c powdered sugar, plus extra for dusting the cake
⅓ c all-purpose flour
5 extra-large egg whites (6 if using large eggs)
3 tablespoon sugar
Preheat oven to 350˚F (175˚C). Use the extra butter to grease a 9 or 10-inch cake pan. Cut out a parchment circle of the same dimensions as the pan and place it in pan. Rub butter on top of that too.
In the oven, toast the nuts until they turn golden brown and aromatic. Allow to cool.
To make brown butter, place the half pound butter in a pan over medium heat. Cut the vanilla bean in half lengthwise, scrape the seeds into the butter, and add the pod halves. Cook for 6 to 8 minutes, shaking the pan occasionally. The butter will initially foam up and then subside and the milk solids will start to caramelize on the bottom of the pan. Pay close attention at this stage to ensure that the butter does not burn. It is a good idea to use a wooden spoon to scrape the solids off the bottom of the pan in the last few minutes to ensure even browning. Allow butter to cool to room temperature (if short on time, transfer to a bowl to speed up cooling). Remove and discard vanilla pod.
In a food processor, combine the cooled hazelnuts with the powdered sugar and grind finely. Add the flour and pulse once or twice to blend. Transfer these dry ingredients to a bowl.
[Feb. 6: Before proceeding with this part of the recipe, I again recommend checking out Shuna's expert hints and tips!] In the bowl of a standing mixer with the whisk attachment (if you do not have one, use a regular bowl and electric beaters), combine the egg whites and sugar. Mix on high speed speed until the egg whites form very stiff peaks, meaning the peaks will hold when you turn the whisk upside down. Be careful not to over whip the whites, however. Transfer the whites to a large bowl. Once the whites are whipped, you have to work quickly.
Using a wide rubber spatula, alternate folding the dry ingredients and the brown butter into the egg whites, about a third at a time. Use as light a touch as possible to avoid deflating your egg whites too much. Make sure to get all the browned bits of butter at the bottom of the pan as well.
Pour this batter into the cake pan and place the pan in the preheated oven. Bake for 50 minutes to 1 hour. Test with a wooden skewer after about 50 minutes. When no moist bits stick to the skewer, remove cake from oven and place on a cooling rack for half an hour. Run a knife around the edge of the cake pan (will not be necessary if using a non-stick cake pan). Invert cake onto a plate, then peel off parchment paper and turn cake back over onto a pretty serving plate.
Dust cake with extra powdered sugar and serve. Chef Goin suggests serving the cake with unsweetened whipped cream and pears caramelized in butter. Recipes for the pears are in her spectacular book, Sunday Suppers at Lucques, which I highly recommend.