Separate eggs, placing yolks and whites into 2 separate large bowls. Let eggs come to room temperature for no more than 1 hour.
Heat oven to 350 degrees F. Grease two 15 x 10 x 1-inch jelly-roll pans. Line pans with waxed paper.
Combine flour and nutmeg on waxed paper.
Beat egg whites until frothy. Gradually beat in 1/2 cup sugar. Beat until stiff peaks form.
Beat yolks with remaining 1/2 cup sugar at high speed until thick and lemon-colored, 5 minutes. Beat in vanilla and rum extract. Fold in flour.
Gently fold yolk mixture into whites until no white streaks remain. Divide between 2 prepared pans, spreading evenly.
Bake in 350 degree F oven for 10 minutes or until cake springs back when gently pressed in center with fingertip.
Meanwhile, sprinkle cocoa powder over length of clean dish towel long enough to turn both cakes out onto. Turn cakes out onto cocoa-covered towel, slightly overlapping two long sides. Carefully peel off waxed paper. Beginning with a 15-inch side, roll up cakes with towel. Place, seam side down, on wire rack to cool.
Prepare Chocolate Cream Filling and Chocolate Ganache.
Unroll cake and towel. Spread cake evenly with Chocolate Cream Filling. Reroll cake without towel. Makes 16 servings.