Bake chocolate cake mix as directed. Slightly cool and invert on a cookie sheet, and place in freezer. (Place on wax paper if you want to transfer back to a cake pan.) While cake is freezing, make filling.
Cook flour and 1 cup of milk over low heat, stirring constantly until thickened. Let cool. In separate bowl beat margarine and Crisco together, slowly adding sugar; beat 4 minutes until fluffy. Add cooled cooked mixture and beat 4 more minutes. Add vanilla near end of beating time. This will look like Cool Whip.
Cut cake into lengthwise and lift top half off in a firm piece, set a side. Place filling evenly on the bottom half of cake and place top half of cake back over the filling.
Fudge Topping: Add sugar, milk, margarine and cocoa in a sauce pan and bring to a boil, stirring constantly. Boil for 1 to 1 1/2 minutes. Let cool slightly to thicken. Pour over cake, cut and serve. Sometimes I cut the cake before I pour on the fudge topping. Fudge will run in the cuts!