Bûche de Noël (French Christmas Log)

My first Bûche de Noël! Miam miam, mes amis!
Bûche de Noël is a traditional Christmas-time cake in France.  It involves a rolled sponge cake filled with a desired filling and is usually encased in a buttercream.  The best bit is that the cake is decorated as a christmas log; so feel free to decorate it with icing sugar for a snowy effect, or even those plastic little christmas trees and deers! Thank you to chef Bruno Richeux for the recipe and for the baking lesson about Bûche de Noël.

Ingredients for Sponge Sheet
4 eggs
100g caster sugar
100g plain flour

Method
In a bowl, beat the eggs lightly to disperse the yolk (by hand or with mixer if needed).  Add the sugar and continue to whisk/electric mix the egg and sugar until the sugar dissolves and pale ribbons form.  At this stage, fold in 1/3 of flour; continue to fold until no flour remains.  Spread the sponge mixture on a tray lined with baking paper.  Bake at 180 degrees (fan-forced) until golden and just firm; perhaps 15 minutes.  The sponge should spring back when cooked properly.  Remove sponge from oven and immediately begin to roll it by rolling it from the shortest edge.  Allow to cool.

Ingredients for Sponge Filling
Chesnut purée, amount as desired (I used 5 tbsp for a thin covering)
Italian Meringue Buttercream (Vanilla Flavoured - Recipe below)

Ingredients for Italian Meringue Buttercream
1/2 cup of cold water 
2 1/4 cups of caster sugar
10 egg whites 
680g soft unsalted butter
1 tsp of pure vanilla paste/extract

Method for Vanilla Itailan Meringue Buttercream
Place the sugar and water in a saucepan.  Bring to the boil.  When the sugar syrup has been boiling for 5 minutes, you need to whisk the eggwhites for the last 2 minutes of this. At the 5 minute mark, remove the sugar syrup from the saucepan; it's finished. Now, with the electric mixer on high speed, slowly pour the hot syrup down the side of the mixing bowl.  When all sugar syrup has been added it should be thick and fluffy. Stop the mixer and change for the paddle attachment.  At this point we need the mixture to cool down.  So, beat on high speed for 10 minutes; or until cool to touch at the base of bowl.  Meringue mixture should still be thick.  Now, return the mixer to high speed and with a knife cut cubes of soft butter into the meringue mixture.  When butter is incorporated, the mixture should be thick and smooth.  At this stage, slow down the mixer to the vanilla.  Now split the italian meringue buttercream mixture in half; placing half in a bowl which will be turn into Chocolate Italian Meringue Buttercream.

Method for Chocolate Italian Meringue Buttercream
In a bowl, fold through sifted cocoa powder until you think it is chocolatey enough.  I would advise close to 1/2 cup of strong, good quality cocoa. 

Method for Assembly
As cake is now cool, unroll the cake from the baking paper.  Chop off any crusty edges. Spread your desired amount of chesnut purée (or other filling eg. curd, coulis, cream, icing) over the sponge evenly.  Now spread all the Vanilla Italian Meringue Buttercream over the chesnut purée.  Roll the sponge now by taking one of the shortest edges of the cake and rolling it forward to meet the other shortest edge.  You should now have a rolled sponge.  Cover the rolled sponge in the Chocolate Italian Meringue Buttercream.  Taking a fork,  lightly draw "bark marks" in the sponge by dragging fork spears from one end of the cake to the opposite end.   On a diagonal slant, cut a 5cm thick slice of cake and place it on top of the cake to create a log effect.  From the opposite end now, on a diagonal slant, do a thinner and smaller wedge and place this on your cake too.  Your Bûche de Noel is now complete! Allow to chill in fridge for 1 hour before serving.