This Passover dessert, made with hazelnuts and matzo cake meal, is rich and delicious and as decadent as it gets.
Heat oven to 350 degrees. Line a baking pan with parchment, and spread hazelnuts on top. Bake until fragrant and toasted, about 10 minutes. Place nuts in a clean kitchen towel, and rub to remove loose skins. Place 1/3 cup nuts in bowl of food processor, and pulse until finely ground; reserve. Roughly chop remaining 1 cup nuts, and set aside.
Grease a 9-inch round cake pan with margarine; dust with cocoa powder. Whisk together remaining 3/4 cup cocoa powder, ground hazelnuts, and matzo meal; set aside.
In a bowl of an electric mixer fitted with the whisk attachment, beat yolks and brown sugar on medium-high speed until mixture holds a ribbonlike trail for 3 seconds when you raise the whisk, about 45 seconds. Fold in 1 cup nut puree from fudge glaze recipe. Transfer to a large bowl; wash and dry mixer bowl and attachment. Place egg whites and salt in mixer bowl; beat with whisk attachment on medium-high speed until soft peaks form, about 1 minute. With mixer running, slowly add superfine sugar; beat until stiff and glossy. Fold egg whites and ground-hazelnut mixture into egg-yolk mixture. Fold in melted margarine. Transfer batter to prepared pan; smooth top with an offset spatula.
Transfer to oven; bake until a wooden skewer inserted in the middle of the cake comes out clean, 35 to 40 minutes. Transfer to a wire rack to cool, about 1 1/2 hours.
Invert pan, and remove cake. Using a serrated knife, slice cake into two layers. Place a 9-inch cake round on a wire rack set on a baking sheet. Place bottom layer of cake on round; spread 1/2 cup fudge glaze on top. Place second layer on bottom layer. Pour remaining glaze over cake, using the offset spatula to cover sides. Refrigerate 30 minutes.
When ready to serve, press chopped hazelnuts onto sides.
Make the nut milk for the Passover Fudge Glaze first; the leftover puree keeps this cake moist and tender.