Preheat your oven to 325°F (163°C). Grease and flour a 10-inch Bundt pan thoroughly to ensure the cake doesn’t stick.
In a large mixing bowl, cream together the butter and granulated sugar using an electric mixer on medium speed until the mixture is light and fluffy, about 3-5 minutes.
Add the eggs one at a time, beating well after each addition. This helps to incorporate air into the batter, making the cake light and fluffy.
In a separate bowl, whisk together the flour, baking powder, baking soda, and salt.
In another bowl, mix the milk, vanilla extract, and dark rum.
Alternately add the dry and wet ingredients to the butter mixture, starting and ending with the dry ingredients. Mix on low speed until just combined. Be careful not to overmix.
Pour the batter into the prepared Bundt pan, spreading it evenly.
Bake in the preheated oven for about 1 hour, or until a skewer or toothpick inserted into the center of the cake comes out clean.
While the cake is baking, prepare the rum syrup. In a saucepan, combine the butter, sugar, and water. Bring to a boil and let simmer for about 5 minutes, stirring constantly until the sugar is dissolved. Remove from heat and stir in the dark rum.
Once the cake is done baking, remove it from the oven and let it cool in the pan for about 10 minutes. Then, use a skewer or toothpick to poke holes all over the cake. Slowly pour the rum syrup over the cake, allowing it to soak in.
Let the cake cool completely in the pan, then invert onto a serving plate. Slice and serve. Enjoy your delicious, moist rum cake!