In small bowl mix flour, baking powder, and salt. Set aside. In medium bowl mix butter, sugar, eggs and vanilla extract. Use mixer or blend together until smooth with a wire whisk. Add flour mixture and mix until slightly thick. Slowly add milk until mixture is creamy and smooth.
Heat oven to 350 degrees. Lightly grease a 8x10 or 8x8 glass baking dish. Pour mixture in glass dish and bake for 35 minutes or until a fork comes out clean.
Let cool for 15 minutes. After cake has cooled a little, melt butter in a medium sauce pan on a low heat. Add brown sugar, milk, and whipping to the pan and bring to a boil while stirring constantly. Boil about 1-2 minutes. Remove from heat and add 2cups of powder sugar. Whisk until smooth. Pour over cake while still warm. Let cool. Then serve.