Start by lining an 8x12 inch rectangular cake tin (often sold as a traybake tin) with baking parchment then brush the parchment with a light but even coating of butter. It really does help release the sponge easier than not doing this. Preheat the oven to 180˚C.
Whisk the egg whites to stiff peaks.
Whisk the egg yolks and sugar until tripled in volume, sift over the cocoa and fold it in until well mixed.
Fold the egg whites into the cocoa mixture then pour gently into the prepared tin and level with a spatula. Bake for 11-12 minutes until well risen, dry to the touch and springing back when lightly pressed in the centre.
As soon as you take the cooked sponge out of the oven, turn it out onto a second piece of parchment, peel off the backing paper and cover with a clean tea towel until cool.
Beat the butter and icing sugar until light and fluffy then beat in the melted Caramac for the buttercream. Don't worry if it curdles a little, turn up the speed and continue to beat, dribbling in enough recently boiled water (1-2 tsp) until you see the buttercream turn soft, smooth and creamy. This is a top tip from cake decorators!
Remove the tea towel from the chocolate sponge, check it's cold and spread with the buttercream in a thin and even layer.
Slice the sponge into 8 evenly sized smaller rectangles (4x3 inches each) and roll each up with light pressure starting at one of the shorter sides. Don't worry if they crack a little. Use a piece of the spare parchment to place the mini rolls on and refrigerate for 30 minutes.
When fully chilled, fill two piping bags with the milk chocolate and the Caramac and drizzle over the mini rolls. As they are chilled, it helps them set faster but in hot weather you will want to pop them in the fridge for 15-20 minutes before serving.
Traybake tins are meant to be 8x12 inches in size but they can vary quite a bit. You can use a tin up to about 9x13 inches with the same amount of ingredients here - the rolls will come out slightly longer but otherwise, you will still get a good shape. If you don't have a traybake tin, a Swiss Roll tin or a roasting dish may work - get your tape measure out!