Make the cake Preheat the oven to 350°F. Grease a 5- to 6-cup bundtlette pan with baking spray and use a pastry brush to evenly coat the details of the pan.
In the bowl of a stand mixer fitted with the paddle attachment or in a large bowl and using a hand mixer, cream the butter and sugars on medium-high speed until smooth, about 2 minutes. Reduce the speed to low and beat in the eggs, one at a time, until smooth. Add the rum and vanilla.
In a medium bowl, whisk the flour with the baking powder, salt, cinnamon, and nutmeg.
With the mixer on low, slowly add the flour mixture, alternating with the eggnog and beginning and ending with the flour, until smooth. Spoon the batter into the molds, filling only ½ of the cavity, and smooth the top. Gently tap the pan on the counter to remove any air bubbles. Bake the cakes for 20 to 25 minutes, or until a toothpick inserted in the center comes out clean. Let cool in the pan for 5 minutes before inverting onto a cooling rack. Let cool completely.
Make the glaze In a medium bowl, whisk the powdered sugar with the eggnog. Add more eggnog 1 tablespoon at a time until you’ve reached your desired consistency (less eggnog will yield a more opaque glaze). Whisk in the nutmeg and salt.
When the cakes are completely cooled, spoon the eggnog glaze on top, letting it drip down the sides. Before the glaze sets, garnish with the sugared cranberries and rosemary sprigs, if using (see tip below). Serve.