Preheat oven to 200 degree C. Grease a 10 inches x 12 inches Swiss roll pan; line base and long sides with baking paper, extending the paper over edges.
Beat 3 egg yolks and 2 whole eggs and caster sugar in a medium bowl for 5 minutes, until pale and very thick.
Pour the hot water down inside the bowl; add chocolate followed by melted butter; gently fold in the flour and baking powder until just combined.
Beat egg whites in a bowl with an electric mixer until soft peaks form. Fold egg whites into chocolate mixture into two batches, until just combined. Spread mixture into pan.
Bake the cake for 12 minutes or until golden and sponge springs back when pressed lightly with a finger.
Meanwhile, dust a kitchen towel, larger than the pan, generously with extra sugar. Turn hot sponge onto the kitchen towel. Peel away the parchment paper gently from the sponge.
Working quickly and using towel as a guide, roll the sponge from a long side. Cool for 10 minutes. Unroll the sponge, remove towel, cover with a clean towel, cool.
Combine strawberries, sifted icing sugar and half of the sumac in a medium bowl. Cover and refer iterate for 30 minutes.
Beat mascarpone and cream in a bowl, until firm peeks form.
Unroll the cooled sponge; spread with cream mixture. Top with half the strawberries. Refill sponge tightly to enclose the filling. Serve topped with remaining strawberries, sprinkle with remaining sumac.