Preheat oven to 180⁰C. Mix egg yolks, caster sugar, 3 tablespoons of warm water, and beat it till it gets foamy. In a separate bowl, beat the 4 egg whites. Add the flour and baking powder gradually into the egg yolk mixture, and mix well. After mixing, add in the egg whites.
Line the baking pan with baking sheet and spread the dough in the pan evenly to 1 cm thickness. Bake for 10 minutes. Remove the cake from the pan, and place it on a damp towel. Roll it to shape and leave it to cool. Simmer a pot of water, and then break chocolate into smaller pieces and place them into a heatproof bowl. Sit the bowl over the pot of simmering water (a bain marie) and allow the chocolate to melt, stirring occasionally. Add the softened butter into the chocolate.
Unroll the cake, and spread the chocolate mixture over the entire surface of the cake. Leave some of the mixture to be used as cake frosting. Roll the cake into the shape of a log. Spread the remaining chocolate mixture over the top of the cake.
Make designs on the frosting using a fork (Optional).
Place it in the fridge to cool before serving.