Step 1Toast the hazelnuts in a 350-degree oven until they smell nutty, about 8 minutes. While they are still warm, rub them with a damp towel to remove as much of the peel as possible. Let cool. Reduce the oven temperature to 300 degrees.
Step 2Grind the hazelnuts in a nut grater and toss lightly with the flour (this can be done in a food processor, but the texture won't be as fine). You should have about 2 cups of grated nuts. Reserve 1 tablespoon to use as garnish.
Step 3Butter and flour a 9-inch springform pan. Separate the eggs and beat the yolks with half the sugar until they hold a 1-second ribbon, about 1 minute with a hand-held mixer. Warm the egg whites over hot water or swirl them above a gas flame until barely warm, then remove from the heat and beat, using clean beaters, with the cream of tartar and salt until they just hold stiff peaks.
Step 4Beat the remaining sugar into the egg whites until the whites hold stiff peaks again. Drizzle the vanilla over the egg-white "sand." Spread the egg yolk mixture over that.
Step 5Sprinkle one-fourth of the nut mixture over the yolks and partly fold together. Sprinkle on another fourth of the nuts and partly fold them in. Continue until all the nuts have been added and fold them in completely.
Step 6Spread evenly in the prepared pan and bake until a toothpick inserted in the center comes out clean, about 50 to 55 minutes. Cool in the pan and then remove the sides. Frost thickly with bourbon-flavored whipped cream and sprinkle with reserved nuts.
Step 1While the cake is cooling, beat the whipping cream until it holds soft peaks.
Step 2Whisk in the sugar and bourbon.