Preheat oven to 325 F. Generously grease and flour a 10-12 cup bundt pan. Set aside.
In a large bowl, beat the butter, sugar, eggs, and vanilla together until well-combined.
Scrape down the bowl as needed.
Add the buttermilk, and mix in until well-combined.
Add the flour, baking powder, baking soda, and salt. Stir in until combined but don't overmix.
Pour the batter into the prepared bundt pan.
Bake for 60-70 minutes, or until a wooden toothpick inserted in the center comes out clean.
Set cake aside while you prepare the glaze.
To make the glaze, add the butter, sugar, water, and vanilla to a saucepan.
Heat, stirring regularly, over medium-low heat until the butter melts and sugar has dissolved. The mixture should be just heated to combine but not brought to a boil.
Poke holes in the cake. I like to use a long wooden skewer.
Pour the glaze over the cake.
Let the cake cool to room temperature in the pan.
Run a knife around the edge of the pan, and invert the cake to serve.
Notes
Butter: the butter should be softened but shouldn't be smooshy. It should dent when pressed but shouldn't lose its shape.
Room temperature: The ingredients will combine better if at room temperature.
Glaze: The glaze just needs to be heated enough to melt the butter and dissolve the sugar. It shouldn't be brought to a boil.
Pouring the glaze: Depending on how many holes you poke, you may find that you need to pour the glaze over the cake a bit at a time to avoid it spilling out.