Prep. time: 45 minutes
Baking time: 30 minutes

Making the sponge cake

1. Break 5 eggs into a large bowl. Add the 155 g of granulated sugar. Place the bowl over (not in) a pan of simmering water and beat for 1 minute with an electric beater. Remove the bowl from the heat and continue to beat on high speed for about 20 minutes.
2. Gently melt the butter. Skim off the white froth. Sift the flour over the sugar-egg mixture. Fold in, then add the vanilla sugar and warm butter.
3. Cover a jelly roll pan with parchment paper. Spread the batter evenly over top with a spatula. Bake at 220° C (425° F) for 8 to 10 minutes.
4. Remove the génoise from the oven and carefully remove the cake from the parchment paper by moistening the paper with a brush dipped in water. Cover the génoise with a clean tea towel until it has cooled.

Making the buttercream

1. Meanwhile, make the buttercream. Cook the sugar with 100 ml (6 tbsp.) of water until it forms a soft ball (about ten minutes, or until some of the syrup dropped into a bowl of cold water forms a ball.)
2. Whisk the egg yolks in a bowl, pour in the hot sugar syrup, beating constantly for two minutes. Continue to beat with an electric mixer for about 10 minutes before adding in the butter, piece by piece.
3. Spread three-quarters of the buttercream over the génoise. Roll the cake up tightly. Decorate the log with the remaining buttercream.
4. Draw the tines of a fork down the log to create the look of bark. Decorate as desired. Refrigerate for at least two hours.