Heat oven to 375 degrees F. Coat a 15 x 10 x 1-inch baking pan with nonstick spray. Line with waxed or parchment paper. Coat paper.
Cake. Sift flour, nutmeg and salt.
Beat egg whites and cream of tartar in large bowl until frothy. Gradually beat in 1/4 cup granulated sugar; beat until stiff peaks form. Set aside.
With mixer on high, beat yolks in second bowl with 1/3 cup sugar until thick and lemon-colored, 5 minutes. Beat in extract. Fold flour mixture into yolks. Fold yolk mixture into whites. Spread evenly into pan.
Bake at 375° for 14 to 15 minutes, until center springs back when gently pressed. Sift confectioners' sugar over kitchen towel. Invert cake onto towel; remove pan and paper. Starting from long side, roll cake up inside towel. Cool, seamside down, on wire rack.
Prepare Filling and Frosting. Combine milk, heavy cream and instant pudding mix. Beat on high speed for 2 minutes, until thickened.
Unroll cake. Spread with half of the frosting. Reroll cake without towel. Cut ends off cake, using parallel diagonal cuts. Reserve ends to use as knots. Place cake, seamside down, on serving plate. Place knot on each side of log. Spread remaining frosting over log and knots, leaving all ends exposed. Dust with a little cocoa powder, so log resembles a birch tree, if desired.