Cherry-and-Chocolate Bûche de Noël

Make the cake

1. Preheat oven to 375°F. Line a 9- by 13-inch rimmed baking sheet with parchment paper. In the bowl of a standing electric mixer fitted with whisk, beat egg yolks with 6 tablespoons of the sugar at high speed until mixture is pale and fluffy, and leaves a ribbon trail when whisk is lifted, about 3 minutes. Transfer to a large bowl.

2. Thoroughly wash and dry mixer bowl and whisk. Add egg whites and salt to bowl and beat at moderately high speed until soft peaks form. Gradually add the remaining 6 tablespoons of sugar and continue beating until whites are stiff and glossy, 2 to 3 minutes.

3. Using a rubber spatula, stir one-fourth of the beaten whites into yolk mixture, then fold in remaining whites until no streaks remain. Working in two batches, sift cocoa over batter and fold gently until fully incorporated. Spread batter on prepared baking sheet in an even layer. Bake 18 to 20 minutes, until cake feels springy and dry; rotate pan halfway through baking. Transfer to a rack and let cool for 30 minutes, then unmold cake and let cool completely.

Make the kirsch syrup

1. In a small saucepan, combine water and sugar and bring to a boil over high heat. Reduce heat to moderate and simmer just until sugar is completely dissolved, about 1 minute. Remove from heat and stir in kirsch.

Make the filling

1. In a small, microwave-safe bowl, sprinkle gelatin over water and let stand until softened, about 5 minutes. In a small saucepan, combine milk with vanilla bean and seeds and bring to a simmer over moderately high heat. Meanwhile, in a medium bowl, whisk egg yolks with sugar.

2. When milk comes to a simmer, discard vanilla bean. Very slowly whisk milk into yolk mixture until thoroughly blended. Transfer mixture to saucepan and cook over moderately low heat, stirring constantly, until custard is thick enough to coat the back of the spoon, about 1 to 2 minutes. Strain custard into a medium bowl and let cool for 5 minutes, stirring occasionally.

3. Melt gelatin in microwave for 15 seconds; stir it into custard and let cool. Meanwhile, whip heavy cream until firm. Stir one-fourth of the whipped cream into custard until incorporated, then fold in remaining whipped cream.

Assemble and frost cake

1. Run the tip of a knife around the edge of the cake. Cover with a clean sheet of parchment and a second baking sheet and invert cake. Remove first baking sheet and peel off parchment. Brush kirsch syrup evenly over cake to soak; reserve any extra syrup for another use.

2. Using an offset spatula, spread filling evenly over cake. Scatter cherries over filling. Use parchment to carefully roll cake to form a 13-inch-long log with seam on bottom. Fold parchment over log so the ends meet. Place a ruler against side of the cake and squeeze cake in parchment to tighten the roll. Refrigerate for at least 1 hour.

3. Just before serving, whip cream with confectioners' sugar until the peaks stand up and hold their shape. Place cake on a piece of parchment paper, and frost with whipped cream. Sift cocoa powder on top (about 2 tablespoons is enough to cover cake). Using two offset spatulas, transfer cake to serving platter. Add additional cherries or other decorations if desired. Slice and serve.

Make ahead

The kirsch simple syrup can be made 1 month ahead of time if stored in the refrigerator or 1 day in advance if stored at room temperature. The cake can be baked and rolled a day in advance and refrigerated overnight. Frost just before serving.