Unsalted butter and flour for the cake tin, 12 chocolate macaroons (optional)
Preheat the oven to 180℃ (or 160℃ convection oven)
1,2 Chocolate Biscuit : Place the egg yolks, sugar, finely ground hazelnuts, flour, unsweetened cocoa powder and melted butter into the bowl of a beater. Combine the ingredients on high speed. Whisk the egg whites until stiff, gradually adding half the sugar. Continue beating until stiff peaks form and add the remaining sugar(30g). Carefully fold into the first mixture.
3 Pour into a buttered and floured cake tin (20~ 22 cm diameter).
4 Transfer to preheated oven and cook for 35 to 40 minutes. Remove from the oven, unmould and cool on a cake rack. Lower the oven temperature to 90 ℃. Sugar Syrup : Put the water and sugar in a saucepan over low heat. Stir until the sugar has completely dissolved. Bring to a boil. Dissolve any syrup crystallized on the inside of the saucepan by brushing cold water down from the rim.
5,6 Drain 2 kimchi cabbage leaves and soak in the syrup. Remove and drain again. Spread the leaves on a sheet of parchment paper, transfer to the preheated oven (90℃) and cook for approximately 1 1/2 hours until dry and crisp.
Cut the 2 remaining leaves into small dice and transfer to the syrup. Bring to a blil and simmer over low heat for 30 minutes. Drain the candied diced kimchi cabbage leaves, reserving the syrup.
7Chocolate Ganache : Put the whipping cream into a saucepan and bring to a boil. Pour the hot cream onto the finely chopped cholate, stirring in the butter until smooth. Cool and divide the ganache into 2 portions(2/3~1/3). Set aside
8,9 Slice the biscuit into 3 even layers. Brush the cut surfaces with reserved syrup.
10 Add the candied diced kimchi leaves to the large portion (2/3) of ganache. Top with the last layer, cut-side down.
11,12 Melt the remaining one-third of chocolate ganache. Pour it evenly over the cake to coat and make it shine.
Assemble the chocolate biscuit (sponge) : Spread the chocolate ganache filling evenly between each layer. Decorate with the crispy dried kimchi cabbage leaves and macaroons as desired.