To prepare cake, coat a 13 x 9-inch baking pan with cooking spray; dust with 1 tablespoon flour. Set aside.
Place egg yolks, 2/3 cup sugar, and 2 teaspoons vanilla in a large bowl; beat with a mixer at high speed until mixture is thick and pale, about 2 minutes.
Lightly spoon 2 cups flour into dry measuring cups; level with a knife. Combine flour, baking powder, and 1/4 teaspoon salt; stir well with a whisk. Add flour mixture and 1/2 cup fat-free milk alternately to egg yolk mixture, beginning and ending with flour mixture.
Beat 6 egg whites in a large bowl with a mixer at high speed until foamy using clean, dry beaters. Add 3 tablespoons sugar, 1 tablespoon at a time, beating until stiff peaks form (about 2 minutes). Gently fold egg white mixture into flour mixture. Spoon batter into prepared baking pan. Bake at 375° for 20 minutes or until wooden pick inserted in center comes out clean. Cool in pan 10 minutes on a wire rack. Turn cake out onto a platter with sides. Pierce entire top of cake with a fork.
To prepare sauce, combine condensed milk, evaporated milk, 1/2 cup fat-free milk, and rum; stir with a whisk. Slowly pour milk mixture evenly over cake. Cover with plastic wrap, and refrigerate at least 3 hours or overnight.
To prepare meringue, beat 4 egg whites, cream of tartar, and 1/8 teaspoon salt in a large bowl with a mixer at high speed until foamy using clean, dry beaters. Combine 1 cup sugar and 1/2 cup water in a saucepan; bring to a boil. Cook, without stirring, until candy thermometer registers 250°. Pour hot sugar syrup in a thin stream over egg whites, beating at high speed until stiff peaks form (about 3 minutes). Stir in 1 teaspoon vanilla. Spread meringue evenly over top and sides of cake. Garnish with nutmeg, if desired.