In small saucepan over medium heat, whisk together flour and milk. Cook for 3-5 minutes, making a thick paste. Cool to lukewarm.
While cooling, use stand mixer to cream shortening (or butter), with sugar, salt and cinnamon.
Add lukewarm flour/milk paste, then beat until slightly fluffy.
Mix in vanilla, raisins and nuts.
If making a layer cake, remove 1/3 of mix from bowl. This will be used as the filling for your cakes. If making a sheet cake, skip this step.
To the rest of the mix, add the confectioners sugar. This will be what you use for top and sides of cake, or if making sheet cake, just the top.
This recipe will make enough to frost your layer cake lightly. If you desire a slightly thicker frosting, try making a 1.5x batch of this recipe.
Once cakes are cool, assemble cake with frosting and filling as desired.