1. Preheat the oven to 375°F and line a 12-cup muffin pan with paper liners.

2. In a medium bowl, whisk together the flour, baking powder, baking soda, salt, cinnamon, and nutmeg.

3. In a large bowl, whisk together the brown sugar, granulated sugar, eggs, vegetable oil, sour cream, and vanilla extract until smooth.

4. Stir in the shredded carrots, and if using, fold in the chopped walnuts or pecans and raisins.

5. Add the dry ingredients to the wet ingredients and fold gently until just combined.

6. Divide the batter evenly among the muffin cups, filling each about three-quarters full.

7. Bake for 18 to 22 minutes, or until a toothpick inserted in the center comes out clean.

8. Cool the muffins in the pan for 5 minutes, then transfer them to a wire rack to cool completely.

9. In a mixing bowl, beat the cream cheese and butter until smooth.

10. Add the powdered sugar, vanilla extract, and milk, then beat until creamy and spreadable.

11. Spoon or pipe the cream cheese topping onto the completely cooled muffins.

12. Serve right away or chill briefly before serving.