To make the sponge, preheat the oven to 180 degrees Celsius (350 Fahrenheit).
In a bowl, add the eggs, lemon juice and sugar, and beat well with an electric mixer until the mixture triples its volume, and has a pale yellow colour.
Add the flour and cocoa powder, and use a spatula to combine all the ingredients into a smooth batter.
Line a rectangle baking tray with non-stick paper, spread the mixture evenly, then bake for 15 minutes until the sponge springs back when pressed.
Leave the sponge to cool slightly, still keeping the non-stick paper on, and roll it starting from the long side.
Leave to cool completely while still rolled.
To make the cream, beat the soft butter with the icing sugar, orange extract and milk to get a smooth cream.
Break the chocolate into pieces and add them to a heat-proof bowl set over a pan of simmering water.
Leave to melt, then add the melted chocolate to the buttercream, and mix well.
Divide the cream into 2 parts.
Add the mascarpone to one half of the cream, and mix well.
When the sponge is cooled, unroll it, and spread the mascarpone cream over it.
Roll it again starting from the long side.
Cut about a quarter of the cake, and attach it to one side of the roll.
Spread the buttercream on top, and use a knife to spread it so that the icing looks like bark.
Sprinkle icing sugar over the cake.