Making the Cake

1. Heat the oven to 425 degrees F. Line a 17-by-12-inch sheet pan (called a half-sheet pan) with parchment paper.

2. Whisk eggs, yolks and sugar in the bowl of an electric mixer set over a pot of lightly simmering water. Whisk until the mixture forms a thick ribbon when it's drizzled back over the surface (about 5 to 8 minutes). Remove the bowl from the warm water bath. Beat the mixture with an electric mixer for 5 minutes more.

3. Sift together flour, cocoa and cornstarch. Stir the vanilla into the egg mixture, then fold in the flour by hand using a flexible spatula.

4. Quickly spread the batter evenly over the lined pan until it's about 1/2 inch thick. You may have batter left over.

5. Bake at 425 degrees F for 10 minutes. Remove from the oven.

6. As soon as the pan can be handled, run a knife around the edges to loosen the cake and slide it out of the pan, still on the parchment.

7. Sprinkle some granulated sugar over the top and roll the cake up lengthwise, still on the parchment.

8. Let the rolled-up cake cool. If necessary, secure it with string or a wedge to keep it from unrolling.

Making the Filling and Frosting

1. Make the filling by placing chopped milk chocolate and bittersweet chocolate into a large metal bowl.

2. Heat 1 c. chilled heavy whipping cream until it steams, then pour it over the chopped chocolate. Let the hot cream melt the chocolate.

3. In a separate bowl, beat the remaining 2 1/2 c. chilled heavy cream until stiff peaks form. Don't overbeat the cream.

4. Let the melted chocolate-cream mixture cool for about 15 minutes, then fold the whipped cream into the chocolate in thirds, using a flexible spatula.

5. Cover and chill until needed.

6. Make the frosting by placing butter in a double boiler set over slowly simmering water.

7. Add semisweet chocolate squares and allow the chocolate and butter to melt, stirring often. This won't take very long.

8. Remove the pan from the heat and stir in the 2/3 c. heavy cream.

9. Let the icing sit at room temperature, stirring occasionally as it cools, until needed.

Assembling the Yule Log

1. Carefully unroll the cake and remove the parchment paper. The cake will want to stay rolled; don't worry about it.

2. Spread the mousse as evenly as possible over the inside surface of the cake; a spatula works best for this step. Try not to let the cake crack, but if it does, don't worry: Icing will cover the cracks.

3. Roll the cake back up. It may be easier to add the mousse and roll as you go.

4. Choose the best looking end of the cake and cut off a 3-inch section, making a diagonal cut. This section will be the branch.

5. Place the rolled-up cake on a serving platter, seam side down.

6. Trim a bit from the branch section, so that the branch is thinner in diameter than the log. Eat the extra piece.

7. "Glue" the branch onto the log at a diagonal with a little icing.

8. Remelt a few tablespoons of icing and drizzle it over the branch to cover it. Work carefully - the branch won't be secured tightly.

9. Spread icing on the log, dragging it with the spatula, to imitate bark.

10. Serve the yule log immediately or chill it until needed. If chilled, allow it to rest at room temperature for 30 minutes before serving.

11. Sift powdered sugar over the top and onto the serving platter just before serving.