Preheat the oven to 350 degrees.
To prepare the bundt pan, cover the entire inside of the pan with butter, making sure every crevice is covered. Lightly sprinkle flour over the butter coating, covering the entire inside of the pan. This will ensure your cake will not stick. Set the pan aside to prepare the batter.
In a large mixing bowl, or using a stand mixer, combine the butter and the sugar. Beat them together for two to three minutes, until they are smooth and creamy. Add in the eggs, two at a time, and then mix in the vanilla and buttermilk. (The mixture will look like it’s curdling, and that’s totally normal.)
Add the baking powder, salt, and baking soda to the flour. Begin adding the flour mixture into the batter, mixing in increments, scraping the sides of the bowl as necessary.
Once the batter is well mixed, it will be light yellow, thick, and creamy. Pour into the pan, and evenly spread the batter filling the pan.
Bake the cake in the center of the oven, on the middle rack, for 50-55 minutes. Five minutes before the cake is removed from the oven, quickly begin to prepare the butter filling, by combining the butter, sugar, and water, in a medium saucepan. Stir on medium heat, until the butter is completely melted and the sugar is dissolved. Remove from heat and stir in the vanilla, then set aside.
Remove the cake from the oven and use a skewer, or dowel to poke at least 15 holes around the top of the cake. Pour the melted sugar and butter mixture over the cake, filling the holes, then allow the cake to sit for about 15 minutes, so it can absorb the sauce.
Flip the cake onto a serving dish, to remove the cake from the pan before the sauce thickens and sticks the cake to the pan.
Sprinkle with a light dusting of powdered sugar before serving.