Preheat the oven to 350°F. Line two large baking sheets with parchment paper and set aside.

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

In a large bowl, whisk together melted butter, light brown sugar, and granulated sugar until smooth. Add the egg and vanilla extract, and whisk until fully combined.

Stir in the grated carrots.

Add the dry ingredients to the wet ingredients and mix just until no streaks of flour remain. Do not overmix.

Using a 2-tablespoon cookie scoop or spoon, portion the dough onto the prepared baking sheets, leaving about 2 inches of space between each cookie.

Bake for 10 to 12 minutes, or until the edges are set and the centers appear slightly underbaked.

Remove from the oven and let the cookies cool on the baking sheets for 5 minutes. Transfer them to a wire rack to cool completely before assembling.

While the cookies cool, make the sandwich filling.

In a medium bowl, use a hand mixer or stand mixer fitted with the paddle attachment to beat the cream cheese and butter until smooth and creamy, scraping the sides of the bowl as needed.

Add the marshmallow fluff, vanilla extract, and kosher salt. Mix on medium speed until smooth, about 30 seconds.

Add confectioners’ sugar, one cup at a time, mixing on low speed until incorporated. Once all the sugar is added, increase speed to medium and beat until light and slightly fluffy, about 1 minute.

Refrigerate the filling for 20 to 30 minutes before using if you want extra sturdiness; it will firm up beautifully while staying creamy.

Spread the filling on the flat side of half the cooled cookies and top with the remaining cookies. Gently press to sandwich.