1.Preheat oven to 160°C/325°F. Grease 25cm x 30cm (10-inch x12-inch) swiss roll pan; line base with baking paper, extending paper 5cm (2 inches) over long sides.
2.Beat egg yolks, 2 tablespoons of the caster sugar and rind in small bowl with electric mixer until thick and creamy; transfer to large bowl.
3.Beat egg whites with remaining caster sugar in small bowl with electric mixer until soft peaks form. Fold sifted flour into egg yolk mixture. Fold in egg white mixture, in two batches. Spread mixture into pan.
4.Bake cake about 12 minutes.
5.Meanwhile, place a piece of baking paper cut the same size as pan on bench; sprinkle paper evenly with extra caster sugar. Turn cake onto sugared paper, peel away lining paper. Working rapidly, use serrated knife to trim all sides of cake. Using paper as a guide, gently roll cake loosely from short side. Cool.
6.Make chocolate filling; beat butter, sugar and rum in small bowl with electric mixer until light and fluffy. Beat in cooled chocolate.
7.Unroll cake; spread evenly with 1 cup of the chocolate filling. Re-roll cake, place on plate; spread log all over with remaining chocolate filling. Refrigerate 30 minutes. Serve dusted with sifted icing sugar.