Preheat oven to 350°. Grease and flour a 10-in. fluted tube pan., Cream butter and sugar until light and fluffy, 5-7 minutes. Beat in vanilla; add eggs, 1 at a time, beating well after each addition. In another bowl, whisk together flour, baking powder and baking soda; add to creamed mixture alternately with buttermilk (batter will be thick). Transfer to prepared pan., Bake until a toothpick inserted in center comes out clean, 45-50 minutes. Cool in pan 10 minutes before removing to a wire rack to cool completely., For icing, in a small saucepan, combine butter, brown sugar and cream; bring to a boil over medium heat, stirring constantly. Remove from heat; cool 5-10 minutes. Gradually beat in confectioners' sugar; spoon over cake.