Step 1: Preheat your oven to 350°F (175°C) and line two baking sheets with parchment paper.
Step 2: In a medium bowl, whisk together the all-purpose flour, baking soda, baking powder, ground cinnamon, ground nutmeg, and salt. Set aside.
Step 3: In a large mixing bowl, cream together the softened unsalted butter, packed brown sugar, and granulated sugar until light and fluffy.
Step 4: Beat in the large egg and vanilla extract until smooth and fully incorporated.
Step 5: Gradually add the dry flour mixture to the wet ingredients, mixing on low speed until just combined. Do not overmix.
Step 6: Gently fold in the shredded carrots, along with the chopped walnuts or pecans (if using), and the raisins or currants (if using).
Step 7: Using a cookie scoop or a tablespoon, drop rounded balls of dough onto the prepared baking sheets, spacing them about 2 inches apart.
Step 8: Bake for 12-15 minutes, or until the edges are lightly golden and the centers appear set but still soft.
Step 9: Allow the cookies to cool on the baking sheet for 5 minutes, then transfer them to a wire rack to cool completely.
Step 10: While cookies are cooling, prepare the cream cheese frosting: In a medium bowl, beat together the softened cream cheese and softened unsalted butter until smooth and creamy.
Step 11: Gradually add the powdered sugar and vanilla extract, mixing until the frosting is smooth, light, and fluffy.
Step 12: Once the cookies are completely cool, spread a generous amount of cream cheese frosting on the flat side of one cookie and gently sandwich it with another cookie. Repeat with the remaining cookies and frosting.