In a medium mixing bowl, whisk together the flour, baking soda, baking powder, salt, cinnamon, and nutmeg. Set aside.
In a large mixing bowl, beat the softened butter, brown sugar, and granulated sugar with a hand mixer or stand mixer fitted with the paddle attachment until light and creamy, about 2–3 minutes.
Add the molasses and mix until well combined.
Add the egg, sourdough discard, and vanilla extract. Mix until smooth and fully incorporated, scraping down the sides and bottom of the bowl as needed.
Add the dry ingredients in two additions, mixing on low speed after each addition. Mix only until most of the flour is incorporated; do not overmix.
Add the oats and mix on low just until evenly distributed. If needed, finish folding the oats in by hand with a rubber spatula.
Cover the dough and refrigerate for at least 30–45 minutes, or overnight.
When ready to bake, preheat the oven to 350°F and line baking sheets with parchment paper.
Using a #40 cookie scoop, portion the chilled dough into approximately 35g portions. Place the dough several inches apart on the prepared baking sheets.
Bake for 9–11 minutes, or until the edges are lightly golden and the centers look set but still soft.
Immediately after removing the cookies from the oven, use the cookie-scoot method to reshape them into neat, round circles. Place a round cookie cutter slightly larger than the cookie over each one and gently scoot the cookie in a circular motion until the edges are smooth and uniform.
Allow the cookies to cool on the baking sheet for about 5 minutes before transferring them to a wire rack to cool completely.
Make the Cream Filling: While the cookies cool, prepare the filling.
Beat the softened butter until smooth and creamy.
Gradually add the powdered sugar, mixing on low speed until incorporated. Increase the speed and beat until light and fluffy.
Add the marshmallow fluff, vanilla extract, and salt. Beat until the filling is smooth and fluffy.
Once the cookies are completely cool, pipe or spread a generous layer of filling onto the flat side of half of the cookies. Top with the remaining cookies and gently press together.
Notes
Yield: This recipe makes approximately 10–11 sandwich cookies, or 20–22 individual cookies. The exact yield will depend on the size of your scoops.
Dark brown sugar is recommended for the richest flavor and pairs especially well with the molasses in these cookies. Light brown sugar can also be used if you prefer a more subtle molasses flavor.
Don't skip the chilling step. The rest allows the oats to absorb some of the moisture in the dough, which contributes to the soft, chewy texture. It also helps control spreading. The dough can be chilled for 30–45 minutes or prepared the night before and refrigerated overnight.
Once the flour is added, mix only until most of it is incorporated before adding the oats. The oats only need a brief mix to distribute them evenly. If necessary, finish incorporating them with a spatula to avoid overmixing.
A #40 cookie scoop works well for these cookies and produces a good size for sandwiching. For the most consistent results, portion the dough at approximately 35g per cookie.
Use the cookie-scoot method immediately after the cookies come out of the oven while they're still soft. This creates smooth, uniform edges and makes the cookies easier to sandwich.
The cookies should still look soft in the center when removed from the oven. They will continue to set as they cool, so avoid overbaking to keep the finished sandwich cookies soft and tender.
Allow the cookies to cool completely before adding the filling. Filling warm cookies can cause the marshmallow filling to soften too much.
StorageStore assembled oatmeal cream pies in an airtight container at room temperature for up to 3 days.For longer storage, refrigerate in an airtight container for up to 1 week. Allow refrigerated cookies to sit at room temperature before serving for the softest texture.Unfilled cookies can be frozen in an airtight container for up to 2 months. Thaw completely before filling.