This cake is so incredibly delicious and so festive for the holiday season. With eggnog and rum inside the cake and in the glaze, it's wonderfully rich with flavor.
Preheat the oven to 350°F and grease the inside of a bundt pan with the butter, making sure to get it inside all of the crevices. Set it aside.
Use a spatula to fold in the poppy seeds.
Pour the batter into the buttered-bundt pan and bake in the preheated 350°F oven until the edges are golden brown, the top has risen, and it's beginning to crack, about 45 minutes.
Let the cake cool for about 15 minutes or so in the pan, then place a plate (or platter) that's at least a few inches bigger than the cake, upside down on top of the pan. Invert the cake onto the plate -- do this gently and don't remove the pan from the cake until you can feel that the cake has released and is firmly on the plate.
This cake serves 12 medium-large slices -- if you're serving it along with other dessert options, you would likely slice it more thinly and therefore have more servings.
The glaze will be thick - I like it this way so that it doesn't all seep into the cake and/or to the bottom. If you'd like it thinner, you can add a bit more melted butter.