Fluffernutter Sandwich Cookies

In a medium bowl, whisk together the flour, baking soda, and salt. Set aside.
In the bowl of a stand mixer fitted with the paddle attachment, or in a large bowl using a hand mixer, mix the butter, brown sugar, and granulated sugar on medium speed for 2–3 minutes, until light and creamy. Add the peanut butter, eggs, and vanilla extract, then mix until smooth and fully combined.
Add the dry ingredients to the wet ingredients and mix on low speed just until a soft cookie dough forms. Scrape down the sides and bottom of the bowl to make sure everything is evenly incorporated.
Using a large cookie scoop (3 tablespoons), portion the dough onto a parchment-lined baking sheet. Refrigerate the dough balls for 20 minutes.
While the dough chills, preheat the oven to 350°F (177°C). Line baking sheet pans with parchment paper or silicone baking mats.
Place the chilled cookie dough balls on the prepared baking pans, leaving 2–3 inches between each cookie to allow for spreading. Bake for 12–13 minutes, or until the edges are set and the centers still look slightly underbaked.
Let the cookies cool on the baking sheet for 2 minutes, then transfer them to a wire rack to cool completely before filling.

In a small saucepan, combine the light corn syrup, granulated sugar, and water, stirring until the mixture is well combined.
Place the saucepan over medium heat and cook until the mixture reaches 240°F (116°C) on a candy thermometer, about 6-8 minutes. Do not stir the mixture as it cooks, as this can cause sugar crystals to form.
While the sugar syrup is heating, add the egg whites and cream of tartar to the bowl of a stand mixer fitted with the whisk attachment. Beat on medium speed until foamy, then increase to medium-high speed and whip until soft peaks form.
Once the sugar syrup reaches 240°F (116°C), immediately remove it from the heat. With the mixer running on medium speed, very slowly and carefully drizzle the hot syrup into the whipped egg whites in a thin, steady stream, avoiding the whisk if possible.
Add the vanilla extract and continue beating on medium speed for 3–4 minutes, or until the marshmallow fluff is thick, glossy, and fluffy.

If needed, microwave the peanut butter for 30 seconds until it has a drizzle consistency. Transfer the peanut butter to a piping bag (or use a small offset spatula) and pipe or spread an even layer onto the flat side of half of the cooled peanut butter cookies.
Transfer the homemade marshmallow fluff to a piping bag fitted with an Ateco 869 piping tip. Pipe a generous swirl of marshmallow fluff on top of the peanut butter layer.
Top each filled cookie with a second peanut butter cookie, flat side down, and gently press to create a sandwich.

Storing & Serving:

Room temperature: Store the sandwich cookies in an airtight container at room temperature for 2 days.
Refrigerator: Store the sandwich cookies in an airtight container in the refrigerator for about 3-4 days. Bring the cookies to room temperature before serving for the best taste and texture.
Freezer: Freeze the unfilled peanut butter cookies in a freezer-safe container or resealable storage bag for up to 3 months. Thaw at room temperature, then fill with the peanut butter and marshmallow fluff before serving. While the assembled sandwich cookies can be frozen, the marshmallow fluff may lose some of its light, fluffy texture after thawing.

Note: Metric conversions and nutritional information are calculated automatically. I cannot guarantee the accuracy of this information.