Filling: Line bottom and sides of 17 1/4x 11 1/2 inch jelly roll pan with foil. Bring syrup and salt to simmer in heavy medium saucepan. Beat yolks and egg in medium bowl to blends. Gradually whisk 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, about 4 minutes; do not boil. Pour into large bowl, set in large bowl with ice water. Cool to room temperature, 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 in prepared pan; smooth top. Cover and freeze overnight.

Cake: Position rack in lowest third of oven and preheat to 350. Line another 17 1/4 x 11 1/2 inch jelly roll pan with waxed paper, overlapping sides. using electric mixer, beat yolks, 1/2 c sugar and vanilla in large bowl until pale yellow and very thick, about 5 minutes. Sift 5 tb cocoa over yolk mixture and fold together. Using electric mixer with clean dry beaters, beat whites and cream of tartar in medium bowl until soft peaks form. Gradually add remaining 1/4 c sugar, beating until stiff but not dry. Fold 1/4 of whites into yolk mixture to lighten. Fold in remaining whites. Pour into prepared pan; smooth top. Bake about 15 minutes. Cool completely in pan on rack.

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

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

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