In a large bowl, whisk together the eggs and sugar until pale and thick. (This will take about 10minutes and the mixture should have roughly tripled in volume; the texture will be creamy and silky, and the mixture should leave a trail when the whisk is lifted).
Using a metal spoon, gently fold in the Mix, ensuring it is thoroughly combined.
Transfer the mixture into the prepared tin and spread evenly into the corners. Bake in a preheated oven (190°C/170°/375°F/Gas Mark 5) for 8-10 minutes until golden and firm to touch.
Dust a large piece of greaseproof paper with caster sugar, and turn swiss roll out onto it. Peel off paper from the base of the sponge and trim off the edges.
Lightly whip the cream until it holds its shape; spread onto the sponge, leaving a border around the edges.
Make an incision about 1cm in from the short edge (making sure you don’t cut through the cake), and use this edge to make your first turn when rolling the sponge. Use the paper to help you roll the sponge tightly. Sit the roll seam side down.
To make the icing, melt the chocolate gently in a microwave (or in a bowl over simmering water). Allow to cool for a few minutes.
Meanwhile, whisk together the butter and icing sugar until creamy; add the cocoa powder and milk and whisk in the melted chocolate. Spread over the sponge and make rough indents with the tip of a spoon or fork (for a finer indent). Decorate with festive decorations or simply with a little icing sugar and holly.