Preheat the oven to 200°C (400°F/Gas 6). Grease a 23 × 33 cm (9 × 13 inch) Swiss roll (jelly roll) tin and line with baking (parchment) paper.
Beat the eggs, sugar, orange zest and juice until light and fluffy. Sift the flour and baking powder over the egg mixture; fold in with a whisk.
Spread the mixture into the prepared tin and bake for 10–12 minutes, or until springy to the touch. Remove from the oven and turn out onto a sheet of baking paper. Let cool for 5 minutes then carefully peel off the baking paper and cover with a tea towel.
Melt the chocolate in a bowl over gently simmering water; cool. Whip the cream until stiff. Gradually stir in the chocolate, coffee and icing sugar. Place about ½ cup chocolate cream in a piping (icing) bag fitted with a large star nozzle.
Remove the tin and top sheet of paper from the cake. Spread three-quarters of the chocolate cream over the cake. Using the bottom sheet of baking paper to help keep the roll neat, roll up the cake, starting from a long side.
Spread remaining chocolate cream over cake. Pipe small rosettes on the top. Dust with cocoa and decorate with chocolate flowers and leaves.