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

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

3. In another bowl, whisk together the brown sugar, granulated sugar, eggs, vegetable oil, and vanilla extract until smooth.

4. Stir the grated carrots and chopped walnuts or pecans into the wet mixture.

5. Add the wet ingredients to the dry ingredients and stir just until 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 mostly clean.

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

9. Beat the cream cheese, powdered sugar, and heavy cream or milk until smooth and fluffy.

10. Spoon or pipe the cream cheese topping onto the cooled muffins and serve.