Fold it through using a spatula or a metal spoon until just combined.
Pour into the cake tin and even out the cake batter.
Bake for approximately 10-12 mins, or a knife inserted into the centre of the cake comes out clean.
Take the cake out of the oven, dust the top with ½ the castor sugar and turn out onto a clean dish towel.
Carefully remove the lining and dust with remaining sugar.
Roll gently into a Swiss roll shape in the towel and leave to cool.
For the icing:
Using an electric mixer/beater, beat the butter in bowl for a minute until softened, then add the icing sugar and cocoa and beat on low until well combined.
Add milk and beat for a further minute or two on medium until you have a smooth, soft, silky icing.
To assemble:
Once cooled, gently unroll the Swiss roll and peal off the baking paper.
Spread a thin layer of icing over the cake and roll up once more.
Cut diagonally across a small section at the end of the Swiss roll to make a branch for the log.
Place the longer piece onto a board or serving dish and attach the branch piece to the side of the longer piece and cover the entire log with the remaining icing.