Preheat the oven to 400° F (200° C) and line 12 muffin cups with muffin papers.
In a medium bowl, stir together the flour, brown sugar, nutmeg, cinnamon, allspice, and salt. Cube the softened butter and mix it in using your hands until it is fully incorporated. Set the streusel aside.
Make the Muffin Batter:
In a large mixing bowl, add the flour, sugar, baking powder, salt, and cinnamon. Add these together and stir them until they are well combined, then make a well in the center.
In a medium bowl or a large measuring cup, whisk together the eggs and eggnog, then add the oil and bourbon. Mix thoroughly, then pour the wet ingredients into the well in the center of the dry ingredients. Use a large spoon to stir, making sure all of the flour gets incorporated, but being careful not to overmix.
Use a tablespoon to pour the batter evenly into the muffin cups. Top the batter with the streusel, pressing it slightly into the batter. Bake at 400° F (200° C) for about 15-18 minutes, or until the tops bounce back when touched. Let them cool for about 30 minutes before glazing.
Make the Glaze:
In a small bowl, stir together the powdered sugar and nutmeg. Add the eggnog and bourbon, whisking until it is well combined and there are no lumps. Use a spoon to drizze the glaze over the muffins. They can be stored at room temperature for up to 3 days, or in the refrigerator for 4-5 days.