In a large bowl, combine flour, sugar, baking powder and baking soda.
Add buttermilk, butter, vanilla and eggs. Mix very well.
Spread batter in a greased 10 inch bundt pan.
Bake at 325° for 1 hour, until a knife inserted in the center of the cake comes out clean.
Meanwhile, while baking, combine butter sauce ingredients in a small saucepan. Cook and stir until melted, but do not boil.
Poke holes in the hot cake (still in the pan) with a toothpick or skewer. Slowly pour the butter sauce over the cake and let it soak in.
Cool the cake for 1 hour. When it is no longer hot, but still warm, run a plastic knife down the edges of the cake and flip the cake onto a large plate or platter.