Preheat your oven to 350°F (175°C).
In a large mixing bowl, cream together the softened butter, brown sugar, and granulated sugar until light and fluffy. Add the eggs and eat well. Stir in the vanilla extract.
1/2 cup unsalted butter, 1/2 cup granulated sugar, 2 large eggs, 1 teaspoon vanilla extract
Gradually add the milk and flour, cream of tartar, baking soda, and baking powder to the mixture, mixing well after each addition. This will ensure a smooth batter.
1/2 cup milk, 3 cups all-purpose flour, 1 tablespoon baking powder, ¼ teaspoon cream of tartar
Fold in the raisins gently until they are evenly distributed throughout the batter.
1 cup raisins
Drop rounded tablespoonfuls of the batter onto a baking sheet lined with parchment paper, spacing them about 2 inches apart. The cookies will spread slightly during baking, so keep that in mind.
Bake in the preheated oven for 12-15 minutes, or until the edges are lightly golden and the centers are set. The cookies will still be soft and slightly puffy.
Remove from the oven and let them cool on the baking sheet for a few minutes before transferring to a wire rack to cool completely.
While the cookies are cooling, prepare the marshmallow-fluff buttercream filling. In a mixing bowl, beat the softened butter until creamy. Gradually add the powdered sugar, mixing well after each addition. Add the marshmallow fluff and vanilla extract, and continue to beat until the filling is smooth and fluffy.
1 cup unsalted butter, 2 & 1/4 cup powdered sugar, 1 & 2/3 cup marshmallow fluff, 1 teaspoon vanilla extract
Once the cookies have cooled completely, spoon a generous amount of the marshmallow-fluff buttercream onto the flat side of one cookie. Place another cookie on top, flat side down, to create a sandwich. I like to fill a ziplock bag or a piping bag with the frosting to make this process easier.
Repeat this process for the remaining cookies and filling.
Store the assembled cookies in an airtight container at room temperature for a couple of hours to allow the flavors to meld together and the filling to firm up slightly.