Preheat oven to 325°F (160°C). Grease a 10–12 cup bundt pan very well with softened butter, making sure to get into all the crevices, then dust lightly 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 the paddle attachment, beat the softened butter, granulated sugar, and brown sugar on medium speed for 3–4 minutes, until light and fluffy. Scrape down the sides of the bowl as needed.

Beat in the eggs one at a time, mixing well after each addition. Add the vanilla extract and mix again until fully incorporated. The batter will be thick.

In a small measuring cup, stir together the whole milk and 1/2 cup dark or spiced rum until combined. Ensure the mixture is at room temperature so it blends smoothly.

With the mixer on low, add one-third of the dry ingredient mixture to the batter, then half of the milk–rum mixture. Repeat, ending with the last third of the dry ingredients. Mix just until the flour is incorporated; do not over-mix.

If using nuts, sprinkle the chopped pecans or walnuts evenly over the bottom of the prepared bundt pan. They will become the crunchy topping once the cake is inverted.

Spoon the thick batter into the prepared pan, spreading it evenly with a spatula and smoothing the top. Tap the pan gently on the counter 2–3 times to release any large air bubbles.

Bake at 325°F (160°C) for 50–60 minutes, or until a toothpick inserted into the center comes out clean or with just a few moist crumbs. The top should be golden brown and the cake should pull slightly away from the sides of the pan. Start checking at 45 minutes if your oven runs hot.

Place the pan on a wire rack and let the cake cool in the pan for about 15 minutes so it sets up and is easier to remove while still warm enough to absorb the glaze.

While the cake cools, combine 1/2 cup unsalted butter, 1 cup granulated sugar, and 1/4 cup water in a small saucepan over medium heat. Bring to a gentle boil, stirring often, and let bubble for 3–4 minutes. Remove from heat and carefully stir in 1/2 cup dark or spiced rum, 1/4 teaspoon fine sea salt, and 1 teaspoon vanilla extract. It will bubble vigorously; keep stirring until smooth.

Using a skewer or thin knife, poke several holes all over the bottom of the warm cake while it is still in the pan. Slowly pour about half of the hot buttered rum glaze over the cake, allowing it to seep into the holes gradually.

Place a serving plate or cake stand over the bundt pan and carefully invert to release the cake. Gently lift off the pan. Poke a few small holes in the top of the cake and spoon or brush the remaining glaze evenly over the warm cake, letting it drip down the sides.

Allow the cake to cool completely for 1–2 hours so the glaze can fully soak in and the flavors deepen. Dust with powdered sugar if desired, slice, and serve plain or with lightly sweetened whipped cream or vanilla ice cream.