Preheat oven to 325°F (160°C). Grease and flour a 10-inch bundt pan.

In a large bowl, cream together the softened butter and sugar until light and fluffy.

Beat in the eggs one at a time, then stir in the vanilla extract.

In a separate bowl, whisk together the flour, baking soda, and salt.

Gradually add the dry ingredients to the wet ingredients, alternating with the buttermilk, beginning and ending with the dry ingredients. Mix until just combined.

Pour the batter into the prepared bundt pan and bake for 55-60 minutes, or until a wooden skewer inserted into the cake comes out clean.

While the cake is baking, prepare the rum glaze. In a small saucepan, melt the butter over medium heat.

Stir in the sugar and bring to a boil, stirring constantly. Remove from heat and stir in the dark rum and vanilla extract.

Once the cake is baked, let it cool in the pan for 10 minutes. Then, invert the cake onto a wire rack and poke holes all over the cake with a skewer.

Pour the warm rum glaze over the warm cake, allowing it to soak in. Let the cake cool completely before serving.