Melt the chocolate and butter together using a double boiler.

Whisk the egg whites and gradually add sugar in two parts to create a meringue with soft peaks.

Mix the egg yolks with sugar until the mixture turns ivory and the sugar is fully dissolved.

Add the melted chocolate mixture to the egg yolk mixture and combine well.

Sift in the cocoa powder and cake flour, then mix until combined.

Add the heavy cream and mix until fully incorporated.

Fold in 1/3 of the meringue gently, then add another 1/2 and fold carefully. Finally, fold in the remaining meringue without deflating it.