Maple Walnut Bûche de Noël

Put oven rack in middle position and preheat oven to 350°F. Butter a 15- by 10- by 1-inch baking pan and line bottom with wax paper or parchment. Butter paper and dust with flour, knocking out excess.

Pulse walnuts, flour, salt, cinnamon, and 2 tablespoons sugar in a food processor until nuts are finely chopped.

Beat together yolks, whiskey, vanilla, and 1/3 cup sugar in a large bowl with an electric mixer at high speed until thick and pale and forms a ribbon that takes 2 seconds to dissolve when beaters are lifted, 5 to 8 minutes in a standing mixer or 8 to 12 minutes with a handheld. Fold in nut mixture in 4 batches.

Beat whites with a pinch of salt in another bowl with cleaned beaters at medium speed until they just hold soft peaks. Add remaining 2 tablespoons sugar 1/2 tablespoon at a time, beating, and continue to beat until whites just hold stiff peaks.

Fold one fourth of whites into yolk mixture to lighten, then fold in remaining whites gently but thoroughly. Stir 1/2 cup batter into butter in a small bowl until combined, then fold butter mixture into batter gently but thoroughly. Spread batter evenly in baking pan and rap once on counter to help eliminate air bubbles.

Bake cake until firm to the touch, pale golden, and beginning to pull away from sides of pan, 12 to 16 minutes. Cool cake in pan on a rack 15 minutes, then loosen sides with a knife. Put a sheet of foil over cake and invert rack over foil, then flip cake onto rack and remove wax paper. Cool completely.

Slide cake (on foil) off rack. Spread 1 1/4 cups buttercream evenly over cake and sprinkle with chopped brittle. Using foil as an aid, roll up cake, jelly-roll style, beginning with a short end. Carefully transfer cake with a long metal spatula to a platter and remove foil.

Frost cake with about 1 1/2 cups buttercream, then chill cake until frosting is firm, about 30 minutes. (If you plan to finish assembling cake within 1 hour, keep remaining 1/4 cup buttercream at room temperature. If not, chill remaining buttercream, covered.)

Cut a thin slice from one end of log (to make end even), then, starting about a 1/2 inch in from cut edge, cut a diagonal piece from same end of cake. Arrange piece on side of cake to resemble a cut branch, using a bit of remaining buttercream to glue piece to "log" and cover seam. Arrange shards of walnut brittle decoratively on cake.