Preheat the oven to 350°F (177°C). Grease a 9×5-inch (23×13 cm) loaf pan and line it with parchment paper, leaving an overhang on the long sides for easy removal.
In a medium bowl, whisk together the all-purpose flour, baking powder, baking soda, salt, ground nutmeg, and ground cinnamon. Set aside.
2 cups (240 g) all-purpose flour, 2 tsps (8 g) baking powder, ½ tsp (2 g) baking soda, ½ tsp (3 g) table salt, ½ tsp (1 g) ground nutmeg, ½ tsp (1.5 g) ground cinnamon
In a large mixing bowl, whisk the melted butter and granulated sugar until smooth and fully combined. Add the eggs one at a time, whisking well after each addition so the mixture emulsifies evenly. Stir in the eggnog, vanilla extract, and rum extract, if using.
½ cup (113 g) unsalted butter, 1 cup (200 g) granulated sugar, 2 large eggs, 1 cup (240 ml) eggnog, 1 tsp (5 ml) vanilla extract, ¼ tsp (1.25 ml) rum extract
Add the dry ingredients to the wet ingredients and gently fold with a silicone spatula just until no dry streaks remain. Do not overmix, as this can lead to a dense loaf.
Pour the batter into the prepared loaf pan and smooth the top with an offset spatula or the back of a spoon.
Bake for 50 to 55 minutes, or until a toothpick inserted into the center comes out clean or with a few moist crumbs attached.
Let the bread cool in the pan for 10 minutes, then use the parchment overhang to lift it out and transfer it to a wire rack. Allow the loaf to cool completely before glazing.
To make the glaze, whisk the confectioners’ sugar and eggnog together in a small bowl until smooth and pourable. Drizzle the glaze over the cooled bread and finish with a light sprinkle of nutmeg, if desired.
1 cup (120 g) confectioners' sugar, 2-3 tbsps (30-45 ml) eggnog, pinch ground nutmeg