Preheat oven to 325 F and grease the bundt pan.

Sift together flour, baking soda, baking powder, and salt. Set aside.

Combine buttermilk, vanilla extract, and rum. Set aside.

Cream butter and sugar on high until fluffy, about 3 minutes. Add eggs, one at a time, beating on medium, until each is incorporated.

Add the flour and buttermilk mixtures, alternating between the two. Begin and end with the flour mixture.

Pour cake mixture into pan and tap the pan against the counter to remove air bubbles.

Bake until it passes the toothpick test, about 45 minutes to 1 hour.

Transfer pan to a wire rack to cool for 15 minutes before unmolding the cakes. Let cool completely before glazing.

Whisk together powdered sugar, rum, and milk until smooth. The proportions can sometimes be affected by the humidity in the air, so if it's too runny, add more sugar. If it's too stiff, add more rum or milk. Drizzle over cake and let it rest to harden before slicing.