Preheat your oven to 350 degrees. Line 2 baking sheets with parchment paper. In a large bowl, cream the butter and both sugars. Add the eggs and vanilla and beat until very creamy, about 2 minutes.
1 cup butter, ¾ cup brown sugar, ½ cup white sugar, 2 large eggs, 2 teaspoons vanilla
In a medium-sized bowl, whisk the flour, baking powder, cinnamon, salt, allspice, and nutmeg.
1 ½ cups all-purpose flour, 1 ½ teaspoon baking powder, 1 teaspoon EACH: cinnamon and sea salt, ½ teaspoon EACH: allspice and nutmeg
Add the dry ingredients to the wet ingredients and beat until just combined. Stir in oats, carrots, raisins, and walnuts.
2 ½ cups rolled oats, 1 cup carrot, ¼ cup raisins, ¼ cup walnuts
Drop by heaping tablespoons onto an un-greased cookie sheet. Bake for 12-15 minutes, or until the cookies are just starting to get brown along the edges.
Remove from oven and let cool for 5 minutes before transferring the cookies to a cookie sheet to cool completely.
For the frosting, beat the ingredients in a medium-sized bowl. Once the cookies are completely cool spread the frosting onto the bottom half of half the cookies. Top each with the remaining half of the cookies.
¾ cup cream cheese, 2 tablespoons butter, 1 cup powdered sugar
Notes
It is best to assemble these cookies within a few hours of the time you are going to eat them. If you are going to make them ahead of time store the frosting in the fridge and the cookies in an airtight container at room temperature. Take the frosting out of the fridge 10 minutes before you assemble the cookies to let it soften slightly.