Preheat oven to 325°F (165°C). Grease and flour a Bundt pan.
In a bowl, whisk together flour, baking powder, baking soda, and salt.
In another bowl, cream butter and sugar until fluffy. Add eggs one at a time, mixing well.
Mix in vanilla extract.
Alternately add dry ingredients and buttermilk, starting and ending with flour.
Stir in rum until just combined.
(Optional) Sprinkle nuts at the bottom of the pan, then pour batter over them.
Bake for 50-60 minutes or until a toothpick comes out clean.
In a saucepan, melt butter, water, and sugar. Boil for 3-4 minutes, stirring constantly.
Remove from heat and stir in rum.
While the cake is still warm, poke holes with a skewer and slowly pour ½ of the glaze over it.
Let it sit for 10 minutes, then invert onto a serving plate.
Brush the remaining glaze over the top.
Let the cake cool completely before slicing.
