Preheat the oven to 425 F. Line a muffin tin with 12 paper or silicone muffin cups, and grease the cups with non-stick vegetable oil spray; this will ensure that they peel off the muffins nicely.
In a medium-sized mixing bowl, cream together the butter and sugar till smooth. Add the eggs, beating for several minutes and scraping the bowl, till the mixture is smooth and light colored. Beat in the baking powder, nutmeg, salt, and eggnog flavor or vanilla.
Stir the flour into the butter mixture alternately with the eggnog or half and half, beginning and ending with the flour and making sure everything is thoroughly combined. Spoon the batter evenly into the prepared muffin cups.
Stir together all of the topping ingredients JUST till crumbly. Stir gently, and don't overmix; you don't want to turn this crumbly topping into a solid mass. Sprinkle the topping over the muffins.
Bake the muffins for 20 minutes, or until they're a pale golden brown. Remove them from the oven, and serve warm, or at room temperature.