Preheat your oven to 325°F (165°C). Generously grease a 10–12 cup bundt pan with butter or baking spray, making sure to coat every groove. Lightly dust with flour and tap out the excess.

In a medium bowl, whisk together the flour, baking powder, baking soda, salt, cinnamon, and nutmeg until well combined. Set aside.

In a large mixing bowl or stand mixer fitted with a paddle, beat the softened butter and granulated sugar on medium speed for 3–4 minutes until light, pale, and fluffy.

Add the eggs one at a time, beating well after each addition and scraping down the bowl as needed. Mix in the vanilla extract until the batter is smooth.

In a small bowl or measuring cup, stir together the rum and buttermilk. If very cold, let the mixture sit a few minutes so it is not icy cold when added to the batter.

On low speed, add about one-third of the flour mixture to the butter mixture and mix just until combined. Add half of the rum–buttermilk mixture and mix on low. Repeat with another third of the flour, the remaining liquid, then the final flour, mixing only until no dry streaks remain.

Spoon the batter into the prepared bundt pan and smooth the top with a spatula. Gently tap the pan on the counter a couple of times to release large air bubbles.

Bake at 325°F (165°C) for 45–55 minutes, or until a toothpick inserted into the center comes out clean or with a few moist crumbs and the top is golden and springs back lightly. Begin checking around 42–45 minutes.

Place the pan on a wire rack and let the cake cool in the pan for 10–15 minutes while you prepare the hot butter rum syrup.

In a small saucepan, melt the butter over medium heat. Stir in the sugar, water, salt, and cinnamon (if using). Bring to a gentle simmer, stirring constantly, and cook for 3–4 minutes until the sugar is dissolved and the mixture slightly thickens. Remove from heat and carefully stir in the rum and vanilla; it may bubble, so use caution.

Using a long skewer or thin knife, poke holes all over the warm cake while it is still in the pan, going almost to the bottom. Slowly spoon about half of the hot butter rum syrup over the cake, allowing it to seep into the holes. After a minute, spoon on more syrup, reserving 2–3 tablespoons for the outside of the cake.

Let the cake rest in the pan for an additional 20–30 minutes so the syrup can fully soak in. Then invert the bundt pan onto a serving plate and gently lift the pan away to release the cake.

Warm the reserved syrup if needed and brush it over the top and sides of the cake for a glossy finish. Let the cake cool completely before slicing for the cleanest slices and best texture.