Beat 4 egg yolks with half of the sugar until pale, thick, and fluffy.
In a separate bowl, beat the egg whites until firm. Add the remaining sugar and beat until glossy and stiff.
Gently fold the whites into the yolk mixture from the bottom up.
Sift in the flour and cocoa powder, then fold carefully from the edges toward the center.
Pour in the melted, cooled butter along the edge of the bowl and mix gently.
Transfer the batter to a greased and floured pan. Bake at 400°F for about 30 minutes. Let it cool on a rack, then cool completely.
Slice the sponge into 3 equal layers. The texture should be moist, soft, and not crumbly. If you want, brush lightly with mandarin jam, but it is not necessary.
For the cream, mix the egg yolk with the same amount of water first, then add the sweetened condensed milk.
Cook the mixture over low heat like a custard until it thickens noticeably. Beat the butter with vanilla. Once the cooked mixture cools, add it gradually to the butter, beating after each addition.
At the end, beat in the melted chocolate, or use cocoa powder instead.
Spread the cream over the cake layers. I assembled it in the pan and chilled it for a while.
Finish the cake with glaze. Any chocolate glaze works; this version uses cream and dark chocolate for a smooth finish.