Preheat the oven to 350°F. In a sizable mixing bowl, blend the softened butter with both sugars until the mixture is smooth and creamy. Incorporate the eggs one at a time, mixing well after each addition. Add the eggnog and vanilla extract, stirring until fully integrated.

In a separate bowl, whisk together the flour, baking powder, salt, cinnamon, and allspice until evenly combined. Gradually incorporate the dry mixture into the wet ingredients, adding approximately 1/2 cup at a time and stirring until the batter is uniform.

Prepare either four to five 5x3-inch mini loaf pans or two standard loaf pans by coating them with non-stick baking spray. Evenly distribute the batter among the prepared pans.

Bake for 30 to 40 minutes, or until a toothpick inserted into the center of the loaves emerges clean. Once baked, place the pans on a wire rack to cool before removing the bread.

For added indulgence, consider drizzling the cooled loaves with melted chocolate.