Mix dry ingredients together.
Add wet ingredients except eggs.
Add eggs and beat with a mixer till well blended.
Pour into a greased and floured 9 x 12 cake pan and bake in a 375 degree oven for 30 minutes or until toothpick comes out clean.
Combine the milk and flour in a saucepan. Stir over low heat until a thick paste forms. Remove from the heat, and set aside.
Combine the shortening, sugar, salt, and vanilla and beat with a mixer until creamy and well blended. Add flour/milk paste and beat until fluffy.