For cake, cream together shortening and softened butter and sugar until very light and fluffy.
Add eggs and beat well.
Combine flour, baking powder and salt, and add dry ingredients alternately with milk (3 additions of dry, 2 of milk).
Beat only until well blended but do not overbeat.
Fold in vanilla extract.
Pour into two well greased and floured 10 inch layer pans (or 3 9 inch pans) and bake in a preheated 350 degree (Fahrenheit) oven for about 30 minutes, or until toothpick inserted near center comes out clean.
Remove from oven, turn cake pans upside down on counter on waxed paper.
In a few minutes, remove pans from cake layers and let them cool completely before making the frosting.
When cake layers have cooled, make frosting. Melt butter in a heavy pan over medium heat.
Add brown sugar and continue cooking until it melts, stirring and watching carefully.
After sugar is melted, add 1/3 cup milk and stir in well.
Turn burner to medium high, and bring to a full boil, stirring constantly. Boil for at least one minute, but not more than two minutes.
Remove from heat and pour over powdered sugar in a metal or heat proof bowl.
Beat well for about 10 minutes, adding more milk as needed if mixture is too thick (it will set up and get thicker as it cools).
Mix in vanilla extract.
Fill and frost layers immediately before frosting sets and becomes very hard to spread.