Preheat oven to 350F. Butter two 9" round cake pans and dust with cocoa powder.

In a medium bowl, sift together flour, cocoa powder, baking soda, and salt. Add oil, sugar, vinegar, vanilla and water. Mix just until smooth and divide evenly between pans. Bake for about 25 minutes or until a toothpick inserted into the center comes out clean. Cool in pan on a wire rack for 10-15 minutes. Remove from pans and cool completely before filling and frosting.

In a small saucepan, whisk together flour and milk. Cook over medium low heat until thickened, stirring constantly. Let cool completely.

In the bowl of a stand mixer fitted with the paddle, beat together the cooled flour mixture, shortening or butter, sugar, and vanilla. Beat on fairly high until the filling is fluffy.

In a clean bowl of a stand mixer fitted with a paddle, beat together the butter, cocoa powder, and vanilla. Alternately add the sugar and milk until the desired consistency is reached.

If desired split cake layers in half horizontally with a serrated knife. Place bottom layer topside down on plate. Top with all of your filling if making a two layer cake, use ¼ if making a four layer cake. Repeat as necessary until all cake and filling is used. Frost as usual, slice and serve.