Filling: Line bottom and sides of 17 1/4 x 11 1/2 jelly roll pan with foil or parchment. Bring syrup and salt to simmer in heavy medium saucepan. Beat yolks and eggs in medium bowl to blend. Gradually whisk hot syrup into egg mixture. Return mixture to saucepan. Stir constantly over medium heat until custard thickens and leaves path on back of spoon when finger is drawn across. Do not boil. Pour mixture into large bowl. Set bowl in larger bowl filled with ice and water. Cool to room temp, stirring occasionally-about 10 minutes.

Using electric mixer, beat cream in another bowl to stiff peaks. Fold 1/4 of cream into egg mixture. Fold in remaining cream. Spread mixture in prepared pan; smooth top. Cover with plastic wrap and freeze overnight.

Cake: Position rack in lower half of oven and preheat to 350 degrees. Line another jelly roll pan (same size as first) with waxed paper or parchment, overlapping sides. Using electric mixer beat yolks, 1/2 cup sugar, and vanilla in bowl until pale yellow and very thick;about 5 minutes. Sift 5 Tbs cocoa over yolk mixture and fold. Using electric mixer fitted with clean dry beaters, beat whites and cream of tartar in medium bowl until soft peaks form. Gradually add remaining 1/4 cup sugar, beating until until stiff but not dry. Fold 1/4 of whites into yolk mixture to lighten. Fold in remaining whites. Pour batter into prepared pan , smooth top. Bake until toothpick inserted comes out cleam, about 15 minutes . Cool cake completely on rack in the pan.

Run small sharp knife around pan sides to loosen cake. Sift remaining 2 Tbs cocoa over large sheet of foil. Invert cake on foil, peel off paper. Peel plastic wrap off frozen filling. Invert atop cake. Peel off foil. Starting at long side and using foil under cake as aid, roll up cake jellyroll style. Arrange seam side down on platter. Freeze cake 1 hour.

Glaze: Place cocoa in heavy small saucepan. Gradually whisk in cream, then syrup. Bring mixture to a boil , whisking constantly. Remove from heat, add butter and whisk til melted. Cool to room temperature.

Spread glaze over frozen cake. Immediately press nuts onto top and sides of cake. Freeze until glaze sets, about 30 minutes. Cover and freeze til firm-at least 8 hours or overnight