Bûche de Noël

Build the egg foam. Whisk the 200 g whole egg and 120 g sugar until very pale, thick and voluminous, then fold in the sifted flour and finally the cooled melted butter without deflating the foam. Continue for about 15 minutes. The egg foam falls from the whisk in a broad ribbon that remains visible on the surface for several seconds. The finished batter is even with no dry flour pockets. The aerated egg foam supplies most of the sponge's lift.

Bake the thin sponge. Spread the batter evenly in the lined 30 × 40 cm tray and bake just until the thin sponge is set and lightly coloured. Continue for about 10 minutes. Temperature target: 180 °C. The sponge springs back lightly when touched and the edges are just beginning to colour without becoming crisp. A flexible sponge must not be overbaked.

Roll while warm. Turn the hot sponge onto clean baking paper, peel away the baking liner and roll it up from a short side while still warm. Continue for about 5 minutes. The warm sponge forms a compact spiral without broad surface cracks. Rolling while warm trains the sponge into the final curve before it sets.

Cool in the spiral. Cool the rolled sponge completely in its rolled shape. Leave it for 30 minutes. The roll is fully cool to the touch before any buttercream is applied. Warm cake would melt the buttercream.

Cook the sugar syrup. Boil the 120 g sugar with 40 ml water to 118 °C; while whisking the pasteurized yolks, pour in the syrup in a thin stream and continue whipping until the mixture is thick and no longer hot. Continue for about 10 minutes. The yolk mixture becomes pale and thick and the mixer bowl feels only slightly warm rather than hot. The hot syrup creates a stable pâte-à-bombe-style base; pasteurized yolks add an extra safety margin. Avoid splashing hot syrup and keep it away from skin.

Emulsify the chocolate buttercream. Beat in the softened butter gradually, then mix in the melted chocolate once both components are close to room temperature. Continue for about 10 minutes. Temperature target: 22 °C. The buttercream is smooth, glossy and spreadable with no greasy puddles or curdled grains. Similar ingredient temperatures reduce the chance of splitting.

Fill and coat the log. Unroll the cooled sponge gently, spread a level layer of buttercream over it, reroll without crushing, then coat the outside with the remaining buttercream and score bark lines with a fork. Continue for about 20 minutes. The log holds a tight spiral, the filling stays inside the roll and the outer coating is even enough to mark with bark-like lines. An even filling limits bulges and cracking during rerolling.

Chill for clean slices. Refrigerate the finished log until the buttercream is firm enough to slice cleanly. Leave it for 120 minutes. Temperature target: 4 °C. The coating is firm but not rock-hard and a chilled knife cuts defined slices without smearing the spiral. Chilling stabilizes the buttercream and the rolled structure. Keep the dairy-and-egg buttercream at 4 °C or below.