Preheat oven to 350°F (175°C). Line baking sheets with parchment paper.

1. In a medium bowl, whisk together flour, oats, baking soda, cinnamon, nutmeg, and salt.
2. In a large bowl, cream together butter, brown sugar, and granulated sugar until light and fluffy.
3. Beat in eggs one at a time, then stir in vanilla extract.
4. Gradually mix in the dry ingredients until just combined.
5. Fold in grated carrots and any optional mix-ins like nuts or raisins.
6. Drop rounded tablespoons of dough onto prepared baking sheets, spacing them about 2 inches apart.
7. Bake for 10–12 minutes, or until edges are set and cookies are lightly golden.
8. Let cookies cool on the baking sheet for 5 minutes, then transfer to wire racks to cool completely.
9. Prepare frosting by beating cream cheese, butter, and powdered sugar until smooth and fluffy.
10. Once cookies are completely cooled, spread or pipe frosting onto the flat side of one cookie and sandwich with another.