Make a sponge. Using a hand-held electric mixer whisk the eggs and sugar until pale, frothy and light. Add vanilla extract and whisk again to combine. In a separate, medium bowl, sift together the flour and cocoa powder. Add dry ingredients into the butter and sugar mixture and fold very gently, until they are well incorporated, and no flour streaks remain.

Pour the batter into prepared baking tin and spread evenly using spatula or back of the spoon. Bake the sponge for 8-10 minutes, until risen and firm to the touch.

Place a large piece of baking paper (larger than the baking tin) on a work surface and dust it generously with icing sugar. Gently invert the sponge onto the paper, removing the other baking paper you used to line the baking tin.

Starting with the long edge, tightly roll up the sponge using the paper. Leave the paper inside and allow the rolled sponge to cool completely.

Meanwhile, prepare the chocolate topping for your yule log. Cube the butter, and cream it until pale and fluffy, using an electric mixer. Sieve in the icing sugar and mix to combine. Gently melt the chocolate and add to butter and sugar mixture, stirring well to combine.

Prepare the filling by placing softened butter in a bowl of an electric mixer. Beat the butter for a couple of minutes, until pale and fluffy, then sieve the icing sugar and cocoa into the bowl and whisk all together until well combined and smooth.

Unfold the cold sponge and remove the paper. Spread the chocolate buttercream on top and re-roll the sponge tightly.

Transfer the chocolate topping into a piping bag fitted with a star nozzle and pipe long thick lines along the sponge, making sure to cover the cake completely. Cover each end of your log with icing (or leave it uncovered if you prefer).

Dust the log with some icing sugar, and add some festive decorations on top, if you would like.