How to Make It
Preheat oven to 350° with rack in lower third. Melt butter in a small saucepan over medium heat until solids begin to brown and butter smells nutty, about 10 minutes. Remove from heat, let cool slightly, and add hazelnut liqueur.
Generously butter a 9-in. springform cake pan and line bottom with a circle of parchment paper cut to fit.
Whirl hazelnuts in a food processor with baking powder and salt until finely ground. Set aside.
In a large bowl, whisk egg yolks with honey until thoroughly combined.
In bowl of a stand mixer, whip egg whites with whisk attachment on medium speed until they begin to get frothy, then gradually rain in granulated sugar. Whip until egg whites form a stiff but glossy peak when beater is lifted.
Stir hazelnuts into bowl of egg yolks and honey. Drizzle in browned butter, stirring to incorporate. Gently fold in egg whites, a quarter at a time, until thoroughly combined.
Pour batter into prepared cake pan and bake until deep brown, sides begin to pull away from pan, and a toothpick inserted down through center comes out clean, 50 to 65 minutes. Cool cake in pan.
Remove cake from pan and pan bottom, peel off parchment, and set on a large plate. Dust with powdered sugar, slice, and top with Malted Whipped Cream.
*Toast hazelnuts in a 375° oven until the skins split and crisp up and the nut is golden brown beneath, about 10 minutes. Let cool slightly, then rub the nuts together in a kitchen towel; most of the skins will fall off (a bit of skin is fine).
Make ahead: Up to 2 days, covered and chilled. Bring to room temperature before serving.