• Preheat your oven to 350°F and line your baking sheets with parchment paper.
• In a large bowl cream butter, granulated sugar, and dark brown sugar for about 2 minutes. Scrape down the sides, then add the eggs, molasses, and vanilla. Mix until just combined.
• Add the salt, baking powder, and cinnamon and mix briefly. Add the flour, cornstarch, and quick oats, mixing on low until the dough just comes together. Scrape the bowl once more to make sure everything is evenly incorporated.
• Using a medium 2-tablespoon cookie scoop, scoop the dough onto the prepared baking sheets, spacing about 6 cookies per sheet.
• Bake for 10–12 minutes, or until the edges are lightly golden and the centers are soft and just set.
For perfectly round cookies: While the cookies are still warm, place a round cookie cutter or glass slightly larger than the cookie around it. Gently scoot the cookie in a circular motion to smooth and round the edges.
• Let the cookies cool on the baking sheet for 5 minutes, then transfer them to a wire rack to cool completely.

Marshmallow Frosting

• Add the softened butter and marshmallow fluff to a stand mixer fitted with the paddle attachment. Whip on medium speed for 2–3 minutes, until light and fluffy.
• Add the vanilla and powdered sugar and mix for another minute, scraping down the bowl as needed. The frosting should be smooth, fluffy, and easy to pipe.

Assemble

• Pair the cooled cookies by size. Transfer the marshmallow buttercream to a piping bag and pipe a generous amount onto the flat side of one cookie. Top with the matching cookie and gently press the cookies together.
• Wrap each oatmeal cream pie individually in plastic wrap and let them sit for a few hours or overnight. This allows the filling to soften the cookies and gives them that classic oatmeal cream pie texture.