MAPLE WALNUT BûCHE DE NOëL

Steps:

• 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.