Melt butter and dark chocolate together using a microwave. Stir until fully combined.

Mix egg yolks with sugar until they turn ivory-colored.

Add the melted chocolate mixture to the egg yolks and mix well.

Sift in the dry ingredients (cocoa powder and cake flour) and fold gently.

Gradually add heavy cream in three portions, mixing well each time.

Prepare meringue by whipping egg whites until stiff peaks form, then fold it into the batter in portions.