Whip all of the above on medium speed for 4 minutes. Put in 3 greased and floured 9 inch cake pans (or a 12x12 pan...or, you can put it in a 9x13 pan and make 2-3 cupcakes as well). Don't pour the entire mix into a 9x13 or it might overflow and make a mess.Bake at 350 degrees for about 21 minutes, or until cake pulls away from the edge or toothpick comes out clean.
While cake is cooking in the oven, make the frosting on the stove in a medium sized saucepan. To the pan add the butter and brown sugar. Boil for 1 minute, or until sugar seems to dissolve. Add the hot milk (microwave for a minute or so until hot) and then add it to the sugar/butter mixture. Beat the milk into the mixture until smooth. Add the powdered sugar until frosting is a spreading consistency. The frosting will thicken quickly as it cools, so don't add too much powdered sugar or it will be too stiff to frost. As soon as the cake comes out of the oven, frost with the penuche frosting as quickly as you can. If the frosting becomes too thick, add a little more milk and continue to beat until smooth and creamy.