Dump first 7 ingredients into large bowl, beat on high for 3 minutes or until well blended, do not omit mayonnaise and do not substitute salad dressing Pour into greased and floured 9x12 cake pan and bake at 350 for 30-45 minutes or until toothpick comes out clean. Let cool
While cake is baking , mix the 5 tbs. flour and milk into a pan and stir constantly on med, high until mixture thickens almost into a ball. Take off stove put into large bowl and cover and refrigerate until cold
After cake and flour mixture are cool, mix the butter, and or shortning and sugar and vanilla into misture and beat on high with mixture until frosting doesn't taste grainy from sugar. You can also add a little cocoa if you would like chocolate, We like it without and it tastes almost like whipped cream , spread onto cake Hope you enjoy., It is not a real sweet frosting. I usually make a double batch of frosting because my kids like it that way.