Combine dry cake mix, fudge marble packet and baking soda in a medium bowl; add remaining ingredients. Blend with an electric mixer on low speed until moistened. Beat on high speed for 2 minutes.
Pour batter into 2 greased and floured 9" round cake pans. Bake at 350° for 20 to 35 minutes or until toothpick inserted into center of cake comes out clean. Cool in pans for 10 minutes; turn out onto a wire rack. Cool completely; if desired, freeze layers overnight to make cake easier to frost. Spread vanilla frosting between layers and on top and sides of cake.
Vanilla Frosting: Whisk flour and milk in a saucepan over medium-low heat until smooth. Bring to a boil; cook and stir for 2 minutes or until thickened. Cover and refrigerate until chilled.
In a medium bowl, blend butter and sugar; add chilled milk mixture. Beat for 8 minutes or until fluffy; stir in vanilla.