Pre-heat the oven to 190°C, 375°F, Gas Mark 5.
Grease and line a 18 x 28cm (7 x 11") swiss roll tin with greaseproof paper. Whisk eggs and sugar together until light and creamy and a spoonful of mixture, when lifted with the whisk, leaves a trail.
Sift flour and Cinnamon together and gently fold into the whisked mixture. Carefully fold into the prepared tin. Cook for 15-20 minutes until well risen and firm to touch.
While still hot, turn the cake onto a sheet of greaseproof paper. Place another sheet of greaseproof paper over the cake and roll the cake up tightly.
Leave the cake rolled in the paper until completely cold.
Meanwhile to make the filling, cream butter, sifted icing sugar and cocoa powder together. Add milk and mix well. Carefully un-roll the cake and remove the paper. Spread a thin covering of the filling over the inside of the cake and re-roll.
Spread a very thin layer of filling onto the cake sides and pipe remaining filling to completely cover cake to form a log effect or simply spread all the remaining filling over the cake and fork over to form a pattern. Dust with icing sugar, decorate as desired with holly, robins etc and carefully transfer to a serving plate.
The cake with filling may be stored in an airtight container for up to 3 days or in a freezer for up to 2 months. Alternatively the cake and filling can be frozen separately.